Ah, now that I pull out the factory installed socket plug to see what's under that solder blob, well, as you said, there it is.
Thanks!

How do I install DCC in a On30 T boiler Shay (Item 25699).  I've had it several years and just now getting it out  to run. No circuit diagram came with it. I have to believe this question has been asked before but I can't find it in any topics. Where do I start?