Strange 20 rd steel magazine for Marushin M16 series

This is a strange magazine I found with my other Marushin 20 rd magazines. On the first look it looks and feels like the well known Marushin blued steel magazine, but on the second look I realized it is lacking the metal spacer plate that prevents live rounds from being loaded into it. Well, Marushin did not insert this spacer in their early 30 rd magazines, but they used a full size follower. Here the short follower can move back and forth so the setup does not look reliable. The bolt stop function does not work neither.

I thought maybe someone removed the spacer, but I can not find traces of it. Even stranger, the top is rounded.

I have never seen such a magazine before. maybe one of our Japanese members has seen it yet?

What do you think?

It definitly does not look stable enough to work, does it fit the gun? could it be a real mag with marushin spring and follower.

I believe that this is an Original Marushin 20 round Steel magazine, but as you say it either had the spacer block removed or was a faulty one that passed inspection, to my knowledge there wasnt any Steel USGI magazine's, they were grey anodized aluminium.
The one in the photo shows minor rust, so I assume it's steel?
It is indeed a strange one none the less 😮

It looks and feels exactly like a Marushin, and it is blued steel. It does not fit a real AR-15/M16, only the Marushin.
Really strange.

First I thought someone took the spacer out to load full size carts, but I can not find traces of the spacer. And the round cut-out is strange. Mhm ...

I was looking at that cut-out, does it look to be machined intentionally or filed down?
It looks as if it was done to aid stripping of the cartridge case from the magazine.

Perhaps it is a Marushin mag modified to fit real-size dummy carts (not model gun carts)? That would explain the missing spacer toward the front of the mag.

Still tough to explain the rounded area in the front, but perhaps it was modified to try to fix a feeding issue?