The "61" refers to the fiscal year in which funds were allocated for the purchase of the aircraft. It might have been built a year or two later.
There are KC-135s around with earlier serials - there are 58- and 59- serials still around. There are also 4 with 57- serials based at RAF Mildenhall.
Last edited by EGTE; 12th Apr 2020 at 18:01.
Reason: Added reference to 57-serials.