Brian et al,
I have had a look in my Ford parts books for mk1 and mk2 cortinas including Lotus cortinas -
the standard 1500 crank is 2731e 6303;
Lotus cranks are 125e 6303 for the 4 bolt crank/ropeseal and 3020e 6303 for the 6 bolt with lip seal;
The 125e 6303 crank is only mentioned in the mk1 lotus book;
The 2731e block is only mentioned in the mk2 book and doesn't mention Lotus;
120e Lotus blocks (aswell as standard 1500 120e blocks) are mentioned only in the mk1 book;
3020e blocks are mentioned in both the mk1 and mk2 books.
I expect the 2731e was the latest design of the Lotus blocks AT THAT TIME and those that we rarely see with 2731e and the "L" between the engine mounts is either a Lotus block that someone forgot to grind the number off (who knows if all the later blocks with the numbers ground off DIDN'T have 2731e on it prior to foundry workers getting there?) or from the "seconds/wrongly machined/cast line".
If the block has Lotus numbers on the engine mount then it must have come in a Lotus engined car from the factory, if not (engine numbers aren't in "Lotus" style but in a Ford style) then maybe it was used for something else such as a standard cortina. No reason that the other lotus parts couldn't be added by an owner at a later date.
HTH?
And yes - you can send me the bitsa diff whenever you're ready! Have you got tired of turning the other diff every week or so?
P.S sorry to the O.P. for the thread hijack! Hopefully we'll be back on track!