Odd! I had the exact same problem with the same Lyman mold number.
Years ago I got a used single cavity 257 420 just to see if my Winchester 1982 even liked the design. Some loads shot okay using these cast bullets, but having to make one bullet at a time to feed a repeater was not great. Standard Lyman and Hornady gas checks fit fine and I never even thought about a check NOT going on any bullet I have in inventory.
So, I find a 2 cavity mold for sale and got it.That should double the production! The mold was in good shape for used and made nice bullets. To my great surprise the checks won't fit! No mater what they don't go on. The bullet shank is just too large! I have checks from Hornady and Lyman as mentioned and a few sizes from Sage (aluminum) and none would go on!
I did not know this was even a common problem nor did I know NOE made something to address it until later.
I was going to sell the mold and thought, that's not good - I either disclose the problem or not sell it. I would not want to stick someone whit a bum mold. Hmmm, come to think of it, someone sold it to me..... Hmmm. I guess people do that kind of thing.
Anyway. Having a metal lathe, I dreamed up turning a piece with a bottom that fits into the press ram like the shell holders and the top end of this piece has a button, the right size of course, to expand the GC. To screw into the top of the press I made a 7/8-14 part with a flat faced bottom to squish the check down on the button . It worked great right off the bat. Place a check on the button and raise the ram. Gas checks fit fine and go on square. Just an unneeded additional step.
IF you like the Lyman mold and the bullets from it I would recommend getting the NOE stuff to enlarge the checks. If not, get a reproduction of that Lyman 257 420 from NOE. He makes great molds.