"All Thompson SMG's were purchased through Britain, not direct from US." I think that statement is essentially right, but might be clearer with a little background explanation.
Commonwealth countries bid for allocations of US-sourced materiel through a centralized purchasing commission, the British Purchasing Commission (BPC), which was made up of various sub-offices and had representatives from each bidding nation, plus a liaison from the USA. Bids were centrally collated/tallied and then, based upon the available output from each month's US production of a particular item, an allocation was made to each nation by the BPC (which rarely matched the bid, especially early in the war). (Available output = Production less US Forces requirements, who got first bite of the monthly apple).
The $$ value of the materiel was added to the receiving nation's Lend Lease account, so in the end, it was the receiving nation that 'paid' for it. In Australia's case, there were two accounts: the LL account, and the Reciprocal Lend Lease account: by war's end, the $$ value of the RLL account was around 75% of the LL account, because Australia supplied so much materiel to US Forces.
There were all sorts of policy agreements used in determining how much of an item was allocated, and what percentage was to be new production, or refurbished to as-new standard (in some items, up to 50%). Policy agreements also determined credits and debits for losses en route, too.
Interesting you mention the sale of arms to the UK in the 90s to make room at DNSDC: I was on the periphery of two instances of that around 2005: the exit of remaining stocks of M16A1 and SLR from the small arms warehouse to make room for new weapons, and the clean-out of the comms warehouse. The M16A1 were sold back to the US - I gather the SLRs went to scrap. The comms equipment was put through the 'muncher' and reduced to shredded scrap.
Mike C
PS: Hope you don't think I was inferring that OMC was manufactured at Lithgow (it wasn't): I was referring to the overall Aust military requirement for SMGs. With Owen/Austen decisions to manufacture taken, there was no requirement to divert a proportion of SAF Lithgow from Vickers and Bren production to produce another SMG, of which there were imported stocks on hand anyway.