Ya I've never seen a socket for them exactly. You may have to cannibalize a PMI card, they used individual pin sockets set into the circuit board.

With the Yellow Jacket relays, It would be nice to be able to test the 4 Poles too that MCE used on the HC-ACI board. The part # should be DS4E-M-DC48V

I think the leads would be a good idea too. Even if you just put jacks in so the leads wouldn't be in the way if you weren't using them. Maybe provide for the coil and 1 or 2 poles.