[Boatanchors] How did they "pinpoint" frequencies way back when?

[Philip Atchley]

Hello again,
Something I've wondered about.  Back in the old days of, say WW II.  How did
"intercept ops" accurately set their receivers for expected incoming
messages?  Say that they expected a message from "Agent X" of the French
underground on 6003 KC (I pulled that from the air).  How did the operator
in England know they were actually listening on 6003.  After all, the agent
couldn't risk long "on air" time calling CQ.  (I presume the transmitter
used by "Agent X" was Xtal controlled and could be used to also zero beat
the agents receiver to the same freq).   I'd say Xtal controlled receivers,
but most of the receivers of the era seem to be VFO controlled.  In this day
of everything digital it'd be no problem, but what about the old "drifty"
days?   I know England used the BBC to send coded messages but the French
Underground didn't have that luxury.

73 from the "Beaconeers Lair".
Phil, KO6BB

DX begins at the noise floor!
Merced, Central California
37.18N  120.29W  CM97sh