So similar to Canadian slow-pitch? Of the 4 distinctive features differentiating from softball (arcing pitch, larger ball, smaller field, lack of gloves) slow pitch has the first, may have the second and third depending on league, but always uses gloves AFAICT.

Slow-pitch isn't Canadian? I played in the US and yes it had an arcing pitch. The big difference is that 16 inch softball has a very large and actually soft ball compared to slow-pitch and an extra fielder (short center) and some extra rules around how many batters / fielders you can have for a game.