Multiple companies made reciprocating saws - think Sawzall - in this configuration. I owned a Sears version for a while, but the ergonomics didn't work as well as the D-handle-kind-of-drill shape of the Sawzall and its competitors. If you're not familiar with the sawzall:
I abandoned it as soon as I got a Makita (and later, a Milwaukee) sawzall-style reciprocating saw; not sure which thrift store was blessed with it.
I agree that the clamp is probably missing. It's possible, of course, that McGraw-Edison used proprietary blades of a peculiar size, as Wen did with the Allsaw. If you've got a friend with a reciprocating saw, borrow a blade and hold it against the end of the shaft. If it fits, it might not be hard to fabricate a clamp; only you can say whether it would be worthwhile.
The two pins and two threaded holes look to be for mounting the blade vertically to the saw (when it's upright) or horizontally. It might be awkward holding that saw sideways (particularly in comparison to the sawzall), and changing the blade would let you hold it upright regardless. That said, when used for its highest woodworking purpose, demolition, a sawzall gets twisted around to all kinds of angles as the cutting proceeds, and having to stop and switch blade mounting would really slow down the party.
