Admittedly, I have not done the transplant process to make a 357/9mm convertible SP101. However, I did it with a Redhawk to produce my convertible 357/44 B&D + 357/38spcl Redhawk, and suppose technically the same path I've done 3 different times with Redhawks (transplant a Super Redhawk 454 cylinder assembly into a 45 colt Redhawk = 454C Redhawk). Equally, I did the same process of transplanting take off cylinder assembly into a Taurus 85 to correct an excessive wear and timing issue - sourced through Numrich that time. I've been rebuilding revolvers for many years, these are relatively low level smithing jobs - no smithing required for Rugers, almost all of them drop in and time perfectly, and the resale market value on used parts is very forgiving. On the single action side, the late 1990's and early 2000's saw an aesthetic trend where I made good money sourcing Super Blackhawk and Bisley cylinders and repolishing them to be transplanted into Vaqueros for cowboy action shooters - admittedly these have FAR greater parts availability, but it's mechanically the same job. Sometimes the market was dry and it was simply easier to watch for a good deal on a used Bisley or SBH, then brush down the Vaquero cylinder to match and send it out the door - still used - for about the same price. I've even gone as far as return revolvers to Ruger, claiming the cylinder was lost and paying to have new cylinders made.
I offered advice on how to get what you want without the extreme cost of custom gunsmithing. I can appreciate it's not common, so you doubted the veracity of the process, but I'll assure you, it's been done long before I ever asked about doing so when you challenged the fact THIS IS DONE, I provided more evidence, with a clear script of how to accomplish it. It does take some effort to accomplish, but it's really a matter of a couple gunbroker listings and leaving your house a few times. The net result ends up being HUNDREDS of dollars cheaper than contracting a custom cylinder made by the few top end revolversmiths who take on such work. A convertible 9mm/357 SP101 doesn't exist, and not everyone is satisfied with a Taurus. If a guy wants a convertible 9mm/357 revolver they can work together themselves, then Ruger is really the one place you can reliably transplant cylinders without timing and dimensional issues - so I provided a pathway which will reach the objective, which I've proven does work. You don't have to believe it, you don't have to pursue it yourself, but the fact you didn't know it was possible doesn't mean you weren't simply ignorant.
There are more things in Heaven and Earth, Horatio, than are dreamt of in your philosophy.