As a part of cleaning up the remnants of Max Howden Crystals, I have come across this R.A.A.F Crystal Unit Holder Y110X in its original box.
The crystal unit is the holder only without an actual crystal or frequency assigned. I imagine that this was held in preparation for orders from the R.A.A.F for specific frequency crystals to be made and mounted.
This is a similar crystal holder noted at the Australian War Memorial.
The reference page is https://www.awm.gov.au/collection/C259365
The text at the War Memorial Page includes estimated manufacture dates c.1939-1945 as a World War II technology.