This is the opto receiver board here, part number 520-5248-00:
https://www.betsonparts.com/opto-receiver.html
https://www.marcospecialties.com/pinball-parts/520-5248-00 - CURRENTLY OUT OF STOCK
http://www.actionpinball.com/parts.php?item=520-5248-00 - OWNERS CURRENTLY ON VACATION