I found a scanned copy of the instructions for the drum, which I guess is useful for those who have one of these.https://www.m1911.org/images/TaylorDrumInstructions.pdf
I also found the actual patent, applied for in 1979, granted in 1982, and expired in 1999.https://patents.google.com/patent/US4332097A/en
The instructions do actually state that it holds 29rds, and I have seen reference to that in a few places, but usually it's stated to hold 28rds, possibly it works much better when loaded with 28rds. I've also seen a couple of mentions of 30rds and even 32ds, but I have no idea if you can even fit 30rds in there.
In addition, some further examples of the offset style of drum. The 'tower' on those looks like it's longer (because it has to fit in the grip just the same), potentially this variant could contain the larger 30rds or even 32rds, just by virtue of having more space, but they seem to be listed as 28rds or 29rds all the same. Possibly the higher attributed capacities pertain to this variant, possibly they all just hold 29rds and people are just making assumptions.
I've only found these drums in .45 Auto, not for .38 Super, nor the later 10mm Auto
More interestingly, I found an old auction entry for, get this, 1000 unfinished Taylor drum magazine bodies, with the front lids and all. Didn't have the other guts, the follower, spring, winder, etc, and some of them were rusted (allegedly just surface rust), most were not blued. The seller, in Ohio, apparently had even more of this stuff, and he wanted $5000 for the lot, didn't sell. I guess this must have been the original guy's unfinished stock (possibly the 1994 AWB killed his business).
One wonders at the opportunity of someone getting ahold of this material and then completing these drums some day.
Pictured here is a stainless Colt Combat Commander 1911 with the drum inserted. I like the wide target spur hammer it has.