The A7M2 was not a Zero-Sen, it was the Reppu.
It was a completely different design, there were no Zero parts in it. Unfortunalely for the Japanese there were many delays (I've posted on this before, if you search for "A7M" or "Jiro Horikoshi" in posts by "Karnak" you should find the data you're looking for) in getting it to production. 1-7 production aircraft were completed, depending on the source, but none ever saw action.
It was to have been the successor to the A6M.
IJN alphanumeric names work like this:
A = carrier fighter
6 = sixth such (carrier fighter in this case)
M = designed by Mitsubishi (other comapanies had different letters)
5 = fifth version of the aircraft (1 was almost alwasy prototype only)
b = third sub-version of the indicated version
So the A7M2 would have been:
A = Carrier fighter
7 = seventh such
M = designed by Mitsubishi
2 = second version of the aircraft (first production version)