Below link should help you for the key location and part number. According to that link, the CMM1200 has 4 models, You will need to determine the "Type" to get the correct manuals and part # which should be listed on a decal or tag on the mower.
This link should help you:
http://servicenet.blackanddecker.com/Parts/FindModelNumber

There are models 1, 2, 2A, and 3. Be sure to downlaod the proper model # for reference. It looks like model 1 has a different key than 2, 2A, and 3. Huge price difference from $2.74 for the 1, to $19.47 for the other three models.

http://servicenet.blackanddecker.com/Products/Detail?isId=true&productNumber=CMM1200&selectedType=19044

I hope this helps you.