The Big 4 front mudguard does change the look but apart from that and the engine, it seems to be all 16H to me. Wheel rims could be 18" but these were also fitted to some (RAF, I believe) 16H sidecar outfits.
You've got a pretty complete machine there, leaving you the choice of a full restoration or a 'tidy-up' and recommissioning.
Feel free to post pictures of the parts and I'll do my best to help with identification.
The Big 4 engines are harder to find parts for than the 16H (far fewer made and a shorter period in service).
The Big 4 front mud guard and 4.00 18 tires had me fooled as a novice. I'm hoping that I can get away with just cleaning it up, new tires, and fresh fluids. First I need to figure out why the kickstart lever will not return.
I will be sure to post pics of the spares as I seperate them and i'm sure I will have several more questions needing answer.