Mercury Wetted Relay
The Hallicrafters HA-1 “TO Keyer” uses this relay as a main component in the keyer. Its an odd bird for sure – liquid mercury makes the contact in this relay. I was able to find replacement parts – but they are $120 or so NOS.
Looking at the schematic – yeah, this bugger does look like its the main keying device in the circuit as far as its output to key a transmitter goes. Here is a really nice description of the circuit in the “TO Keyer”, names after Jim Ricks, W9TO (SK):