Did you find info on TM504A? It could not have been produced for very long. In my "newest" 1983 Tek catalog, Tek are offering the TM500 (not A) and they had then brought out the TM5003 and TM5006.
The user customization of the frames was an asset for flexibility but can cause damage with surplus TM500 equipment. People don't check for customization including movable tabs in the card edge connector to block module ingress. Any "new to you" TM500/5000 needs to be checked for a few things. Past users may have made mods so that the frame is not "plug and play" and this can cause you to inadvertently damage module or frame. Worthwhile as you said, to understand the customization possibilities and options.
I am still a big TM500/5000 fan. There wasn't anything like it back then. Working units have uses today. Basic physics measurements are accurately and inexpensively made with TM500/5000 if you don't need computer interface. Easy to repair. Lots of info and parts.
Not sure what would be today's "equivalent" system with the breadth of modules and cases. It was a very flexible system although bulky by today's standard and no computer interface unless you had certain newer modules and frames with GPIB.
George
GeorgeP