My latest intel tells me that the Mk1s were fitted with RR Merlin 22s, and the Mk3s were fitted with Packard Merlin 228 engines which were based on the RR Merlin 22, but complying with American "standards and measures" (whatever that means, maybe AF instead of Whitworth nut and bolt threads) and with magneto and carburettor modifications, and a extensive toolkit. But how can you tell which is which. I always thought that one way to tell the diffenrence is to look at propeller. The propeller was different for the Mk1 and Mk3. The Mk3 having the paddle wheel props, and the Mk1 having (I forget the proper name) the pointy props.
I just found the serial numbers for the 19 lancasters, is it possible to find out which serial belonged to which Mk lanc.