[Mike Cecil]

Darrin,

I have some information on the 3 inch mortar base plates pertaining to Australia:

(1) pre-war and early war base plate Mk.1: as per the image in the Mortar Carrier handbook. Same as the Brit mortar plate.
(2) circa May 1943 introduction of the No.4 Mk1 base plate: essentially the same shape as the Mk.1, but with increased ribs and strengthening pieces to cope with higher incremental charges, as the Mk.1 was found to bend when firing higher charges. Most obvious differences were underneath, where there was substantial increased ribbing (and greater weight!). This appears to be the one illustrated in the book page image posted by Rob Love (above).
(3) In early 1945, certainly before June 1945: introduction of the base plate No.6 Mk.1(Aust), which is the square one with the two rope handles, as seen with mortars issued post-war. The plate was introduced to cope with the increased recoil of higher incremental charges and to be several pounds lighter in weight than the No4 Mk1. This remained the service base plate post-war until the withdrawal from service of the 3 inch mortar.

I'd suggest that finding either a Mk.1 or No.4 Mk1 base plate in Australia will be very difficult. You perhaps might find one overseas.... then you have to get it to Oz .... and past customs.

Regards

Mike