Starrett No. 164 drafting table T square

A kind neighbor just gifted me a Starrett 164 T square that has an odd spring plunger on it. At first doing an image search nothing came up, but a txt version of the 50th anniversary catalog had this;
“cut No. 164 represents a nickel plated T square, with spring tempered steel blade and aluminum head…has an automatic clasping device to hold it…against a metal straightedge … of a drafting board or table…”
Seems rare with no images coming up?

Here it is after cleanup;

One more;

That's a neat one! As the graduations don't continue to the edge, I assume it would have been used with another square that overlaid the rule on this one?

Yes, it's quite the beauty, and cleaned up nicely once I figured out the process to apply.
It's for use with the drafting table arm, so graduations don't need to go to the edge. I also thought it might be missing a sliding straight edge, but the arm remains parallel to the edge of the table, so it's complete!
that catalog was from 1930, so I'm thinking at the start of the depression a fancy accessory like this might not exactly have been flying off the shelves..
it was 7.60 for the 32" version, so not cheap, but I bet that Starrett wasn't making a lot on it either. also, it's measuring function can be accomplished with a regular rule, and its square function with a simple flat square laid against the table edge, so it's kinda redundant.

calling Starrett collectors! any ideas on how rare this is?

just a bump to see if any Starrett collectors are lurking today..

I just picked one up at a tool auction. Paid $250.

I had an inquiry on how I cleaned this up, and I thought I'd post that here for anyone interested.
first, I use the art conservators mindset (done some of that too) of doing the least harm, and preserving surface and in some cases patina if it doesn't harm preservation or function.
this tool has a nickel plated steel blade, and it had a decent amount of rust on it, primarily small pinpoint spots. I had some difficulty getting this cleaned up.
first tried Brownell's Steel White (same basically as Evaporust), but that wasn't working. I think they were "under" the nickel plate a bit.
then went to CLR. of course as I went to the acidic products I was careful to test and be sure I wasn't stripping the plating. not working.
tried some Barkeepers Friend WITOUT scrubbing of course as it is abrasive with oxalic. not much happened with this either.
this was some stubborn rust! so I got out the big guns and gingerly hit it with a phosphoric acid product, RustOleum Rust Dissolver.
finally got somewhere, but there were some larger spots of rust that needed a little help and I spot brushed with a soft steel brush.
once I did this I had some shiny spots, and most of the rest of the blade was dull and not looking too hot.
I VERY reluctantly began to brush the rest of the blade, first I tried a brass brush, but I had to go back to the fine steel brush.
I had expected the steel to abrade the nickel but it was a hard deposit (electroless probably), and it actually burnished nicely.
used that brush on the screws too.

the aluminum T head was easy, bit of detergent and then a superfine grey scotchbrite hand pad.1000 grit paper on the flat sides and working face.
coat everything with food grade machine oil as is my typical practice on hand tools.

hope that is informative and helps, Cy