Piper did not build an L-7.
Found this, 42-88638/88657 Universal L-7A-UN Model Monocoupe 90AF,88655 cancelled, the others delivered to France.
and this, (scroll to bottom)
http://www.centercomp.com/ILPA/index.html
RAF aircraft serial numbers, AF745 to AF944 were for the Vengeance II.
A pic might help.
Regards,
Mike