N1550-A XO (not a typo)

Yeah, they sounded familiar. I acquired a bunch of 615As in the early 1990s that looked just like that Ebay linked unit. Mine had been removed from the ceiling of a Houston sports auditorium during an upgrade, and each had a 70V distribution transformer mounted to a plate on the speaker frame. My old speaker building partner and I removed the transformers and mounting plates and resold the 615As in stereo pairs after mounting them in appropriate BR enclosures with their N1550-A crossovers.
I didn’t think they sounded as good as 604s or separate Altec horn/woofer combos, but they were priced right.