I realised after writing this that you are probably looking for an Omega strap. But still........
Why not try Waccex Accessories. 20/18 in Cognac or Reddish Brown. Seems they have a few, as well as Rios. Really good straps and reasonably priced too.
Easy to slip with any form of modding on a 21mm, rotary tool or knife. I've tried before and failed.
But, you could try some of those very cheap square edged leather punches. I did a CF strap recently that did not have a tang slot. Worked really well with 2 taps of a small engineering hammer. Mind you, it was in the centre of the strap, so had good shouldering. Taking off 0.5mm each side is probably too small and after all, half a mil is probably easy to squeeze in between the lugs. Might make getting the spring bar in and out a bit of an effort though.
Good Luck.
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