UNIBUS Mystery Board ID

This hex-width board was found sitting in a UNIBUS backplane. It looks like it may have more than two PCB layers. The metal handle, where the board number is usually stamped, is unmarked; no board number is stamped there. On the front side, top left, is "AD413A MADE IN USA". The top middle had the "200415-03" handwritten on it, but that has been crossed out by a marker, and "200850-02" appears beneath it. The top right has "S/N 5781" handwritten. There are two 50-pin BERG (maybe formatted Pertec) connectors at the front top. On the back of the board, at the top right, is "MRX42 200255-02", machine printed. A little to the left of that is "3183", handwritten. Google has returned nothing. What have I here? -- Jeffrey Sharp The email address lists(a)subatomix.com is for mailing list traffic. Please send off-list mail to roach jay ess ess at wasp subatomix beetle dot com. You may need to remove some bugs first.