Quite interesting Peter
Since the Models 75A-1 to A-3 were serialled in one single serial range, No 1 to 335, the model 75A-4 started all over again with No 1
No 21 (A-1) was destroyed with GC II/5 already on 20/09/1939 so this has to be No 21 (A-4) which has no fate in the Curtiss H-75 book published by Lela Press. Many of its sisters were captured by the Luftwaffe , ie Nos 16,17,18,19,20, 22, 23, 24. No 30 used the code KQ+ZA, so we can see the Stkz is also very valid.
Thanks again Peter
Cheers
Stig