It's probably a 4mm pitch M39 as used on some ( like mine ) Hare & Forbes supplied lathe.
although the supplied docs do not show it, it is possible to cut the 4mm pitch thread as I have made several backplates for different chucks.
I've not used it but have seen a supporting post mounted under the front end of the cutting tool to stabilize it in the vertical direction.
It might help solve your problem.
Fred.
Here's my solution to a caliper that constantly flattened its battery, piggybacked a AAA cell on the back with hot glue and ran some wires to the battery connections.
I have the same mill, also in Oz, branded Optimum, it has metal gears in that position. The only problem I've had is a motor controller failing but that was easily replaced. I suspect the gears are module 1.
Cheers, Fred.