Thanks gents.
The comments on the .38 Richards barrel being heavier than the RM 1851 profile (presumably an 8" vs 7.5") was definitely new to me, as was the fact that a .38 RM barrel can mount on a .38 1871 OT frame, and that open tops are slightly lighter than the same caliber/barrel length/grip frame equivalent RM. I had suspected that the .38 barrels would be heavier, but as I couldn't compare the outside measurement of the round vs octagon barrels, it's difficult to even guess at weight without handling all of the varieties.
Last time I was at Taylors they had a whopping two open tops/conversions in stock, only one of which was a .38.
One useful thing I did discover was that Uberti has addressed the ejector rod problem in the .38s by changing to an ejector rod that tapers to a narrower diameter in the middle of the rod, instead of using the same straight rod used with .45s that would hit the cylinder wall. One of the gals there was able to demonstrate that .38s could now be ejected without fiddling with the cylinder, though she mentioned that there was probably still plenty of stock with the old .45 rod style remaining. No sign of them in their parts array yet, however.