Welcome to APUG.

According to this manual for a slightly newer Opemus 6 Standard they made 4 lenses with that smaller M23.5 x 0.5 thread:

Belar 4.5/50 #392 812 110 262 (that you have now, but hazy)
Anaret 4.5/50 #392 812 10 239 (don't confuse with Anaret S that has Leica M39 thread)
Anaret 4.5/30 #392 812 110 271
Anaret 4.5/80 #392 812 110 245 (don't confuse with Anaret S that has Leca M39 thread)

I'd search the internet for one of those. I think the later enlargers used a larger lensboard that may not fit your enlarger.