The actual quote is on page 288 of the reprinted "Sixguns by Keith", at the end of paragraph 4. He's talking specifically about the 45 Colt, and concludes with "If I had to shoot only factory ammo the rest of my life, I would take the 45 Colt as my game and defense cartridge". He mentioned in another place that the 44 spl, when handloaded far exceeded all other available cartridges in killing power. He felt it was double the killing power of the 357.
In the same chapter, "Selection of cartridges" he wrote regarding the 44 spl "The cartridge has been badly handicapped all its life by being sadly underloaded. The factory load of a 246 grain slug at 770 feet is no more than a light target load and a disgrace to so fine a cartridge. (end of p 281-start of p 282). He went on describing other attributes of the cartridge and loading it to its potential. This was all before the 44 mag came into existence. Keith took to the 44 magnum right away when it was introduced. He had been trying to get people at Smith, and at the ammo companies to come out with a modernized 44 spl load, telling them the he and many other had used the heavy loads in the Smith and Colt guns for years without issue. They exceeded his intent, but he liked it.
Back to the 44 spl. He started loading various cartridges up, experimenting with them in the early 30's or before. The instance where he had a 45 Colt cylinder let go, coupled with the wide variation in chamber throats and chamber sizes, and bore sizes, convinced him to stick with the 44 spl for his work with heavier loads, as it had heavier cylinder walls, and was also more consistent in dimensions. He had worked up heavy loads in the 38 spl, the factories came out with the 38-44 (on the 44 frame, a 158 gr bullet @ 1100 fps). Keith worked up heavier loads, and his correspondence with people at Smith & Wesson likely was at least partially responsible for the 357 magnum coming out. He liked it, but felt it still wasn't as good of a killer on game as the heavy 44 spl loads. The early 357 loads were a 158 gr bullet @ approx 1550 fps in an 8 3/8" barrel, a fair bit hotter than the current factory loads. as far as I know, he didn't blow any guns up working with the 44 spl, and his loads were pretty commonly known and used.
As was mentioned in the other thread, by the 50's, the loading manuals of the day had them listed as standard fare for the 44 spl. The only guns available were the N frame Smiths and Colt SAA's. both of which seemed to do quite well on the heavy loads, without serious issues that I've ever heard of.
Someone mentioned here, or the other thread, that "the guns weren't meant for such heavy loads". Well, unless one has insight into the actual thinking at the factory at the time, I'd suggest that's a hard assumption to make. The factory loads may have had X pressure specs, but they were making 357's on the same frame that ran much higher chamber pressures than the heavy 44 spl loads. The frames were quite capable. The factory was also quite aware that there were many people using heavy loads in their 44 spl guns. The fact that there were no known problems with the Keith level loads over a fairly long period of time, it may be safer to reason that the guns were in fact built to take them.
Just as Ruger builds 45 Colts capable of far exceeding the factory level loads (and we have a pretty good idea exactly what they will take), it was pretty common knowledge to use the level of loads Keith and others worked up. The 1950's NRA manual I have shows pressure tested loads from factory level on up to the Keith level. I believe they were in older Ideal and perhaps Lyman manuals also, but I don't have any to check (just looked, my 1970 Lyman No 45 manual shows 17.5 grs 2400 with the Lyman No 429421 bullet, giving1150 fps in a 5 1/2" barreled Colt SAA with the then current solid head cases). When the newer solid head cases came out in the mid 50's, it was discussed, and loads worked with to keep them in the same ball park, this wasn't guess work, this was HP White Laboratories testing loads.
It may be noted that when the 44 magnum came out, Keith worked up handloads, and mentioned that the factory loads were hotter than his, and he was happy to keep it that way, as they had the pressure testing equipment to work those loads up. He was satisfied with his loads being below factory, in pressure and velocity. This seems somewhat at odds with those that felt he was a radical and pushed everything to the limits and didn't care if he broke things in the process. I believe he felt that the practical upper limits had been reached, and that we then had very capable guns and loads at that point. Before the work was done with the heavy 38 and 44 spl loads, it really couldnt be said that we had much in the way of high performance loads for handguns for hunting or defense use. BTW, the factory 357 loads were a lead swc, modified from the Keith type bullets, and tended to lead barrels badly. We didn't have the nice jacketed hollow point loads of today, unless one was a bullet caster and cast their own hollow points.
It was mentioned that Speer tested his level of loads and came up with about 28,000 psi. I'm curious as to the details. The bullets matter, the cases matter, as anyone that reloads is aware. The earlier test information shows pretty consistently that the loads of 18.5 grs 2400 showed about 1200 fps and ran right about 20,000 psi in the balloon head cases. When the new cases were tried, the old load showed about 28,000 psi. It was generally felt that being back in the lower-mid 20,000's was better. The NRA manual showed a load of 16 1/2 grs 2400 as a appropriate load with the new cases, though it sowed a bit more pressure than the older cases, and a bit lower velocity. Keith settled on 17 1/2 grs, knowing it was higher pressure than the old load, but was willing to live with the 25,000 psi pressure readings.
Hope this long post was of some interest.
