Can anyone ID my master cylinder?

[FarmTruk]

Many years ago (20+), I recall helping my father change the MC on the '68 SWB 307 2bbl Auto C10.

The brakes (4 wheel manual drums) are scary bad, and I know that new cylinders, hardware, and shoes are in order. Also, a flush and likely turn the drums, maybe even new rubber lines. All that seems simple enough. But if I've got the wrong MC, then I guess that goes on the list too.

However, after looking at pics of dozens of engine compartments on this site, I've yet to see a MC exactly like mine.

As far back as I can remember, the brakes have always pulled to the ditch side, but it seems to have gotten steadily worse over the years. That's why its been taken off the road. They're scary bad.

The proportion valve seems correct, as far as I can tell, but the MC looks wrong. There is a bolt that holds the cover on, as you can see in the pics. All the ones I've seen before have wire hoop clips to secure the top.

I do not know where this came from, and there are no markings or #'s that I can use to ID.

Anyone seen this before?