I can go and check your old metal holder tonight to see if it works with generic 23mm LNBs.
Went looking for metal CM feed parts in the garden house.
I have to admit that I couldn't find the metal feed holder.
It's in there somewhere, not just sure exactly where.
But I did find a CM feed-horn, and a 23mm universal LNB (Inverto).
Here's a picture of the neck of the CM feedhorn:
Also found my digital caliper(s). And a new LR44 battery.
The "neck part" (size A on picture) of the CM feed-horn is 22.3 mm in diameter.
The metal rings add to this, making it (size B on picture) 29.3 mm in diameter where they are.
For comparison, the plastic neck of the Inverto is 22.9 mm in diameter.
As the rings of the CM feed-horn fits snugly into the recesses of the metal holder, I conclude that normal 23mm feeds should fit into the metal feed holder.
The rings on the CM feed-horn adds keying and a fair bit of support.
A heavy, non-CM feed may suffer from the diminished support that the metal CM feed holder provides.
Nevertheless, as the holder is made from metal, it should still work.
But not with 40mm LNBs...