you must have an earlier Omega, I think Quan made the cart slot a bit wider in the later revisions because the 161 multicarts were not fitting in it.
When Quan made the Omega he only measured the width of the early release carts, he didn't know that SNK widened the newer carts by about 2mm. That's most newer carts you had to kind of squeeze them in and on the 161 is was a bit wider than that. You can easily mod your Omega and widen the cart slot as I did before, you just need a dremel and a steady hand. I think Quan was also selling the newer top shells with the widened slots.
It's actually the opposite. Aside from boot shells, the earlier carts were wider than the later releases. I used a Metal Slug cart as a reference thinking all the carts were the same width.
Ahh, yes, now I remember. I made the same mistake on one of my early CMVS units as well.
Strange, all my original carts from NAM-1975 and Magician Lord (oldest) to SSV and MS5 (newest) fit perfectly in my Omega, the only carts that had tight fit were the 161, Razion and Gunlord. Even Knight's Chance fit well, however I enlargened teh slot a bit for the wooden carts to fit better.