[Kurt S]

I have 3 Holley baseplates with the squared off corner. What were these originally used on?
#1 and 3 have 172 throttle plates. #2 had 109 and 311 plates.

#1 & #2 have side mounted PCV.
#1 has a small rounded boss and hole on the left side and a port on the lower right mounting boss.
#2 has an extra unused vacuum boss at the bottom, both right side mounting bosses are slight different than #1, extra hole by bottom left mounting boss.

#3 has rear mounted PCV and looks similar to #1 except the PCV and the boss & hole on the right side.

What's mounted in the very center of #2? The other 2 have a hole there.
What models got the squared off mounting boss?

Thanks!