The lens probably had severe spherical aberration (overcorrection) in its normal configuration due possibly to poor workmanship. Luckily for you, the reversed configuration with flint leading has added the reverse amount of spherical aberration (undercorrection) that just cancels out most or all of that in the lens. I have seen a few amateur-made lenses that would not work in their normal mode, but worked reasonably fine when reversed.
Rolando
Rolando
