Hello Thom,
Referring to the 'Original MGA' book by Anders Ditlev Clausager ; the MGA's all used a prefix number 15G or 16G. The numbers obviously refer to the engine's capacity, and the 'G' being an abbreviated from MG.
The same basic 1489cc B-series engine was also used in the MG Magnette ZA, the Morris Oxford Series II, and the Austin A50 Cambridge. I might only speculate that your engine '15C' originated from the latter for 'Cambridge'. (see page 55)
Your second letter 'N' is usually a 'U' on an MGA, which refers to 'centre gearchange', or else 'Da' which is for a close-ratio centre-change gearbox (page 69).
And the third letter 'H' refers to High Compression versus 'L' for Low (supplied to countries where fuel was of low octane).
The supposition that your B-series engine is not of MGA origin or specification may be supported by its very high engine number ..of over 134-thousand, whereas the MGA engine numbers (including both 1550 and 1600cc) only went up to 31,660 (page 59).
Please Note. I am not in the least an expert, I just happened to have the MGA book at hand - so I looked it up for you.
I hope it helps,
Bfg