Seen a 45-70 cartridge made by USCco on the internet that had a cannelure about 3/4 of an inch down the brass shell. Bullet appeared to be approx. 500 grain. Anybody tell me what that cannelure represents? Thanks.

Black powder bullets did not have a crimping cannelure on the original bullets, as they sat directly ontop of the powder column.
That avoided bullet set back under recoil in lever action arms.
When smokeless was introduced it became a problem and the bottom cannelure was put there to stop bullet setback.

You can find that on 45LC also.

Thanks, Chickenthief! Tom from MN

It was also a visible clue to the use of smokeless powder use & was also used in I think, three lots of Frankford Arsenal loading, again tinned case.

Thanks Pete!