I have my Dad's Uniflow from the same time era, 1967. On the box that it came in there are two check boxes, one for rifle the other for pistol. Our box is hand checked "Rifle" which is what my Dad purchased it for and that was it's use was until I got a hold of it. That Uniflow has, as would be expected, the large cavity metering drum.
When I started using it for pistol loads I found that some powders metered well, others not so well. It has been years since I ran the tests so I don't remember which powders and the +/- tolerance but I had some tolerances even with AA#2, more than what I thought was acceptable. Bullseye forget it. So I opened up my wallet and purchased the pistol drum for a newer Uniflow that I use for pistol and keep the older one set up for rifle.
If you look at and compare the cavity size it will be apparent to most handloaders why using a smaller diameter cavity would be of benefit to smaller charge loads. Of course I'm not the measure of all things handloading and I don't pretend to be. However, this is a forum where posters on a regular basis advise beginners not to use Titegroup because the gap between the "start with" and "not to exceed" load is very shallow and I have noticed that basically no one but me will step in and say that we should exercise all due caution with our loads and if we do that we can use Titegroup without worry. Part of that "due caution" is the use of the proper metering device.
I would never in a million years presume to tell a handloader with 50 years of experience that they have been using an ill advised system for all that time. However, I would also not state on a public forum that it is ok to use a metering cavity that may be very well shallower than the thickness of the individual kernel of the potential powder being used. My point is that for $30.00 you get the proper tool for the job. True, out on the interstate I have fixed my car with a pair of pliers when I didn't have a 10 mm box wrench. But does that mean I should sell all of my 10mm wrenches and advise others to do the same?
Am I being overly cautious? Maybe. Is it worth spending $30.00 to have the proper tool for the job? I think so. Any one of us could forgo spending the almost $100.00 for a Uniflow and purchase a Lee measure for less than what RCBS sells the pistol drum. It really comes down to exactly what it is we expect out of this hobby and our personal tolerance for chance. Personally I think that the one place to spend money on especially for pistol shooters is on powder handling. But that's just me.