darwins - Brian Hardy's Surface Stock File discusses the M and N stock as part of an early LPTB plan to integrate the Met and District more closely when the District was extended to Barking, with both types being based around the recent L stock. Although the N stock was for the District and the M for the H&C, he doesn't mention a change in couplers when the Ms became the Q35s (although he does mention some other changes - e.p brakes, air doors). .
The M stock was a close copy of the previous L stock (except the L stock had a slight beading line around the doors) built in Feltham by the UCC. There must have been some compatibility coupling method between the 1905 saloon stock and the M stock.
The M stock did however have EP brakes and several complete trains had air doors and passenger door control which must have been a real change for the crews!