This is my old Commander, and I can't find anything definitive on the rear sight.
Its like a Novak but has a smaller dovetail and a step at rear.

Am wondering if this could be cut for a Bomar.

I'm no help on the replacement, but that sight was called the Enhanced Colt Officer/Combat Commander sight.

Yeah Defenders had it too.
The smaller dovetail has me thinking it might be used for a diff sight w some machining.
Its not the big Novak dovetail.

Bought this gun new, was an Enhanced Series 90 Commander.
I added a Pachmayr arched MS housing and Wilson guide rod, Hogue slabs.
Its been a champ, I just never liked the sights.

The trigger is stock and is very good (why I bought it).

With the existing rear sight, you cannot modify that slide for a Bo-Mar.

To my eyes, that little dovetail is probably for the sights used on Series '80 Gov't Models, then behind that dovetail it was flattened and the notch cut for the Novak-esque sight that's on it now. The Series '80 sights were basically the old GI sights made taller, for better visibility.

I might be completely wrong about it, but that's what it looks like to me.