It is a Elmdene Power Supply, for the two offboard RIOs that I have, an external sounder and a landline voice communicator.
The voltages on those RIOs (Option 61 - Diagnostics) were a bit higher (RIO 2 = 13,96V; RIO 3 = 13,80V) than the voltages on the onboard RIOs 0 and 1, which had voltages...