Hi there Adam --
I think that pretty much any of the older letter-type plug-ins should work with your 545A.
There's everything from strain gage plug-ins, to interesting ones like the Type O, ( operational amplifier ).
Here's another reference site that may be of help to you -- http://www.barrytech.com/tektronix/vint ... ntage.html
Scroll down to roughly mid-way of the page, LOTS of stuff on the plug-ins !!
Best of luck !