Commandos were produced with a twin-leading-shoe drum brake between 1968-1973ish. I'm not sure off the top of my head when they started producing them with a disc on the front, but generally speaking the drum brakes are more desirable from a collectors point of view. And to be perfectly honest I always thought the Norton TLS was a better brake than the disc anyway, but that's only my experience.
Also, it's not too hard to retro-fit earlier commandos with disc front-ends, but you need to track down all the correct fork parts and brakes which, whilst not uncommon, can be expensive to source.
Good luck with the project