The Magnecraft "W77" relay was discontinued 30-plus years ago. There was at least one second source that was compatible, but not since the early 1980s.
The more-plentiful 'ice-cube' size relays you can still buy just won't fit the original socket. This is why Barkett sells you a new socket and a relay, both. It's all he can obtain.
We adopted a policy of using a 12-Volt relay when it's necessary to replace a W77 that's genuinely worn out. Only takes two rectifiers and two filter caps to 'borrow' the 6.3-Volt AC power for the tubes' heaters. The installation is a bit more complex, but eliminates the shock hazard from the mike wiring. After all, the new relay socket is wired in a very different pattern from the old one. The additional trouble to do it this way is not so much in comparison.
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