Finally there is an alpha numeric designation. A letter followed by a number. These designate modifications to the thing. "A" means it's an official modification. E means the modification is experimental. The number following is how many modifications have been done.
For instance, the AH-64 Apache helicopter. Variants include the AH-64A (basically the original version), AH-64B, AH-64-C, and the AH-64D. Using sequential letters rather than the "A#" system seems to be fairly common for airframes, and "experimental" and "prototype" designations are contained in the first part of the designation, i.e. for instance the prototype for the AH-64 was called the YAH-64A and it's competitor was the YAH-63A.