Just to check you are aware the IXMOO will fit the IIIf ?
Slight drop in the film gate, which a washer will fix, as it is smaller.
Having said that the FILCA are usually cheaper.
I found best bet was to ring around dealers, who usually can't be bothered to list them, but have box of bits such as this, only don't tell them what they asking on e-bay
The late IIIfs and IIIg don't need a washer their base plate has a locating finger.