What is this?

[Dumpstick]

Not all factory wad cutter rounds had the double cannelure’s.
Some I have.

Not every brand does it. Winchester and Remington do.
Federal has a single cannelure, towards the middle of the brass. The Federal brass with a cannelure towards the top is not wadcutter brass.

 

[fatcpa]

Not a big deal, but not all wadcutters are hollow based. I have 2 moulds, one is flat based and one is double sided...seats either end up.

 

[Dumpstick]

Yeah, I've got an old Ideal (pre-Lyman) 358495 4-cavity. Button nose square base 145gr wadcutter. I'm thinking of getting a Lee 6-cav for the tumble-lube wadcutters, just because it's a 6-cav. And, tumble lubing is so easy.

Love the 4-cav Ideal, it makes 'em quick. I load them in .357Mag brass somewhat hotter too, makes for a thumper at 1000fps.

However, when I do get a box of the Hornady hollow base, I like to load them in the wadcutter brass. I probably don't shoot well enough to notice a difference, but it makes me feel better.
Too, the factory loaded them in special brass for a reason. Back in the day, Rem and Win would load tens of thousands of the wadcutters for competition. They wouldn't have bothered with WC brass for no reason. The thinner brass lets the bullet slide in without distortion of either the brass or the bullet. The very light powder charge makes the slightly thinner walls of the brass a non-issue.

 

[Dumpstick]

Also, to the OP -
RemUMC headstamp is for the Remington Union Metallic Cartridge Co. As you probably figured, it is no longer extant. I see the brass from time to time. Wadcutters are loaded so light, the brass lasts forever. Most folks don't even bother crimping, and I have loaded them without even re-sizing. Just push the WC in with my thumb.

I have some very much older 38 Spl brass, that has large pistol primer pockets. This is from before the time that SPP was standardized, and the companies just made them however they pleased.