How to fix Bar Clamps that Slip?

My grandparents are moving, and are organizing stuff... I just found an antique bar clamp (An ELZETT, a bit smaller than that one though).

The sliding arm/jaw wouldn't slide on the bar, so I removed the rust.
The sliding jaw wouldn't lock onto the bar when I tightened it. I tried enlarging the grooves/notches that were on the bar with a hacksaw, but that didn't really help. 

Does anyone have an idea for making it lock/catch on the bar? My grandpa might have several more of these clamps!
Drilling holes into the bar and adding a removable pin - no, because this steel is really hard and I'd go through 10 drill bits.
The only thing I can think of is to take it apart, and use the hardware for a homemade wooden clamp. But I think it would be easier to start from scratch...

I really want to fix this clamp! The quality of old tools, throat depth of this clamp, the screw, the swiveling pad - I need these!

Yonatan24 (author) 8 days ago

The notches were very shallow/small, to begin with, unlike the $8 150cm bar clamps that I bought at a hardware store. But they have their issues too, the bar is too thin and bends. See step 7: https://www.instructables.com/id/How-to-Make-a-Woo...

I couldn't find any info online about this subject!

Yonatan24 (author)  Yonatan248 days ago

How about building some type of wooden cage around it that will hold it in place?