scopes unlimited model 525

I was given this oscilloscope from a guy saying that I know along with some other Heathkit ones I'm pretty sure these are all pretty old but it seems to work but it is missing one of the channels for the plug-in module and it'd be really cool to find one that's compatible if you know anything about these Scopes drop a comment down below I am curious if the plug modules are actually made for more than one type of scope or not