I think the calibrators were intended for use inside radio trucks, rather than anything where the set was exposed to the elements or liable to condensation dripping on it.
The British used a separate wavemeter (Class 'D' No.1) which was still not waterproof (initially, though there was a later version that was immersion proof when the lid/accessories container was clamped on). They were really only issued to control stations in higher formations during WW2 as far as I can make out: everyone else used the netting call from the control station who would pick a clear spot as close to the assigned frequency (on his dial) as possible.
Frequency inaccuracies were the main reason for post-WW2 sets being designed for crystal control, not to mention simple operation because the user might be getting shot at while trying to contact the net.
Chris. (G8KGS)
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