This looks like any other basic BB B/C/E body motor mount bracket, except for the extra hole.

Is this for some kind of special application, or did somebody with a drill make a custom mount?

That really looks like that super secret MM from the skunkworks down near Area 51. On the other hand it looks like someone drilled it, that would be the right side mount and there doesn't appear to be anything there that would require that hole.

I've seen chain used on horsepower monsters to keep the motor from ripping out of it's mount...

was that hole drilled or was it punched out like the rest of the holes?

maybe as guy did this one too..LOL

Check out the *upper* set in this pic I found in the for sale section. It has the same hole, and they're listed as BB A body..?

Pretty sure pass side are the same ..driver side is the funky one on an A body

I will have to check my collection of A-Body big block mounts.

The upper right in the picture is the driver's side motor mount. It bolts on to the two holes in the oil pump flange and the front ear on the side of the block. Some B-RB blocks have the bosses but not drilled and tapped and some don't even have the bosses. Some C-body cars used the same set up and that's how I got a 440 for a good 361 and a $125 back in the day.