a point of curiosity.
In comparing WW 45 LC cowboy loads with recently acquired UMC 45 Colt with original "U" marked primer i notice That the casong length and rim design differ significantly with the UMC brass being about a quarter inch shorter and having no groove forward of the casing rim...
When examining the cylinder chamber of a 1909 pattern revolver I see a head spacing(?) Rim towards he front of the cylinder. Clearly the UMC will not rest on this rim. The SAAMI diagram shows a min-max spread of casing length of only 0.075"...
any advice?

A lot of antique cartridge brass is a bit different than original spec now. Most have a bit of extractor groove that the original form didn't have, and the rim/head area is a bit different since they're not balloon head brass anymore.

45 Colt headspaces on the rim, nothing forward of it. Those grooves in the cylinder may be for headspacing 45ACP using clips, if what you're describing is what I'm picturing as a little lip in each chamber.

that sounds right..

it just seems odd to have brass so much shorter..

Nominally shorter is perfectly acceptable in a revolver.

Shorter doesn't really effect anything much in a revolver chamber.... to a limited extent. For instance shooting 38 specials in a 357 magnum. However, do it long enough with hot loads and you can get a burned ring in the cylinder where the case ended.

Buy some half moon clips and you can shoot those 45acps.

Bull!