I thought the 1404 plates were invar, but not the 1403. Have to check that.
Don't know for the 1403, but the 1404 and 1406 are invar.
I don't think there is any aluminium in the 1404 ... haven't found some
Our plan is to polish the invar plates and maybe have them (and the bolds,spacers...) gold plated
, more important is the removal of the tin/solder plating of the brass(?) housing parts to prevent whisker. Maybe will be tricky for/around the throug-hole plating ... But a very good workshop and galvanic is inhouse
also vacuum/ Nx refill / clean room/ temper all there... but time and motivation ....since we have nice&stable working 1404 .
I attached a picture of one of those and marked a wisker... probably posted before ..