Well, the manual says NB which I believe stands for normal blow, that is, not slow blow, not fast blow aka fast acting, but normal. I think this is the same as what some manufacturers call medium acting.

So, I guess NA would mean normal acting or something like that?

Anyway, if you can't find one that is rated NB, you can usually safely put in a FB (FA) but you may get the occasional blown one at machine power up as the NB ones would be designed to withstand some surge current. I would be reluctant to use a SB fuse as a replacement; that could be asking for trouble down the track.