my dad tells me they were used on J3 piper cubs ( which are still around ) in 1941......
he has found something in writing about this but is subject to correction.they were used with other engines at the time such i think as continental. they may well have been used on more than the cub...
but, you should enquire...
would'nt it be nice to find they could be fitted to a cub as you can still get J3s...i am sure cub owners will answer you , but as i think the american forces may have discontinued the franklin engine in the J3 in 1941 there may not be much information around now as to its use...
anyway...good luck with it...
the dean.