12 speed astronic medium duty in the 63 plate and older models, 12 speed astronic the big version on the 64 plate onwards.
If you look at the engine MX12.9 has a one piece black plastic rocker cover, the PR engine has two small alloy rocker covers, the MX11 also black plastic but only about 3 inches deep, the MX12.9 is a good 6 inches deep.
Used to see a lot of Daf tippers pre 2006 they had the XE engine (bigger version of the PR) good tough lump like the odd head gasket, then they went onto the MX12.9, I only ever saw one with hub reduction and HD rear susp it was one of them silo erection trucks, pretty rare beast different rear springs and enormous diffs/axles.
The fact it liked to be revved suggests to me its the PR engine.