Hi PGF - the 707-200 was an increased thrust version with 15,800 lb P+W JT4s, which were basically the civil version of the J75 engine. First flown June 11 '59, into service Dec. 20 '59. Only customer was Braniff with 5, so they were -227s, one of which was crashed on a customer acceptance flight. No other interest because the airlines were happy to wait for the turbofan version while Braniff needed some short term that could deal with the hot and high conditions on their South American routes. The other 4 went to BWIA then various other operators before being broken up between '77 and '84.
Re. the customer suffix system, after 21 to 99 were used, 01 to 19 came next then the alphanumerics. A0 to Z9, then 1A to 9Z, then double letters, AA to ZZ eventually. Not all intermediate combinations are used, for instance its A0, A1, A3, A4, A6, no A2 or A5, which I believe means there once was a customer assigned code A2 (or whatever) who then cancelled.
Regards - Musang.