Familiar with this sight/scope mount?

Came across this M1 video showing an attachement near the clip latch?
Does anyone know this mount?

It's an M1C scope mount without scope...

Thanks Jim. I did some reading on that and see how the C's D's were mounted. Interesting read.
Says John Garand didn't like three holes in his receivers.
The C version reminds me of Luepold QRW.

There are lots of threads here about those two also...lots of guys here have them.

The M1C mount requires five (5) holes in the receiver, three for screws and two for pins.

As soon as I posted that, I remembered the pin holes. haha.:)

I wrote an article for the Spring 2013 GCA Journal titled "M1C vs M1D - Was The Fix In?" It seems pretty clear that it was.