[ARC5] Crystals in WWII

[Robert Eleazer]

I always thought it quite odd that so much US WWII equipment used a crystal to manually tune a VFO to the correct frequency.  Did they do this as a back up, so that the equipment could be tuned without a crystal, in case crystals were not available?  

Gordon wrote about the prewar exercises in which crystal distribution was found to be an unexpected nightmare, leading to the design of the ARC-5 series with VFOs.    

Or did they design the other military sets for VFO and then ended up using crystals because production caught up to demand and they were available?

Wayne