bats; fast and slow

[Little Jimmy]

Fed fastpitch. Before tonights game, partner(field) and I(plate) discussed rumors of one player getting away with using an illegal bat.. Both of us say we will be dilligent. I check all bats using my handy banned bat list. All OK. 4th inning, girl comes up using a bat I approved in pregame ( I had been watching each one carefully ). She smacks a killer homerun. As soon as she crosses the plate, partner comes to me and says "check the bat". I say no need, its one I approved. He says "but that's a slow pitch bat". I say it doesn't make any difference, it's not a banned bat. He says a slow pitch bat (labeled as such) cannot be used in a fast pitch game. I let it stand as is but we had a friendly discussion afterwards with both of us doubting our positions.

The question: is it legal to use a slow pitch bat in a fast pitch game? ASA doesn't seem to draw a distinction between the two.