My G15CS/1235xx was dispatched in July 1967. Chances are the N15CS motor in question left the factory in July or August of 1967, too.
An Atlas motor can be fit into a Commando frame, especially a late 1967, as it came with a distributor and Amal concentrics. The first 3500 or so Commandos (1968 and early 1969) used the Atlas-style distributor and the tach drive in the timing cover, the same as the N15CS.
The head would need to be a Commando, or modified, as the N15 used Atlas-style rocker lubrication. Commandos moved the rocker feed to the sides of the head because of the different head steady. The Ni5 head is not cast with the proper boss on the side of the head for this modification.
Primary mounting is a little different, as the N15 used the AMC primary with Atlas clutch, but fitting a Commando primary is not difficult.
Balance factor is different between the two applications, but who's to say if the engine was re-balanced to the Commando balance factor?
Sounds like somebody blew a Commando motor 40-something years ago, and found another Norton 750 motor to replace it.