[Collins] 75A-2 question

[W5vhp at aol.com]

A friend of mine asked me this question:
 
 
   
I was looking at the 75A-2 before chucking it to ebay and noticed a plug in  
unit in the NBFM socket.  
Vector Electronic Co. P/n 1210 jj and 5935-553-2840


Do you know what it is?  
I've looked on Google but didn't come up with anything. 


If you have any ideas, please reply to me.
 
 
 
    Thanks and 73,
 
    Larry      W5VHP