In about 1990 I joined the Vintage Motorcycle Club In Victoria, Australia and at that time we had a lot of WW 11 ex servicemen in the club. One guy was a dispatch rider and said that when Darwin got bombed in February 1942 he would ride an M20 from Darwin to Adelaide mostly unsealed road to relay messages. His biggest threat was from US convoys going towards Darwin taking up the whole track and from the dust storm that they created. He loved the M20 so much that after the war he raced them and opened a motorcycle shop in Box Hill. We also had Military Police who rode the Harley Davidson WLA later on in the war. I agree most WM 20 in Australia have matching engine and frame numbers and often are not on the War memorial Transport records from Canberra. On another note a lot of Merlin engines were left here after the war still in crates and some of these soldiers family helped pull them to bits to seperate the steel studs etc from the alloy to be recycled. They said the perimeter fence was 2 crates high with the engines still in them to keep intruders out. Another guy purchased airforce instruments as they were auctioned of in the 1960's at very cheap price and later sold them on mainly to USA. My wifes uncle was a mechanic in Darwin working on Spitfires during the war and later worked on race boats here in Australia which were powered by Merlin engines. One boat called MISS BUD won quite a few races around 1976-1981 with its powerful merlin engine. He later travelled the world working on Merlin engines and when in his eighties knocked back lots of offers to travel to fix Merlin engines.
The enormous standing Army in Australia seems to have been in place for a relativly short period whilst Japan was threatening to invade.
Its possible that the M20s in Australia visited the workshops for repairs just as much as anywhere else but because the workshops only had one or a few machines in at any time they were fixed and returned to their units with original engines, the mass engine changes in the UK, Germany and other places are more likely to be post war or in workshops full of machines where a production line system was used.
We did the same as every other army
When machines came in for major service it was swap the engine / gearbox then back out ASAP
When they sold off the WDB40's about 1/4 of them had mismatched engine & frame numbers which became a problem latter on when we tried to get them registered
Some where I have a copy of the order for B40s.
Went some thing like
495 bikes ( 500 including the 5 test bikes ) + 40 engines + 20 complete frames + 20 complete front ends
The rest of the spares never made it as they got sunk in the Suez canal
