Never cared until you asked, since I know Americans strangely dismissed the Amiga very early. (Except for a select few who had intelligence to match their taste and computer knowledge.)
For those who didn't have immediate use for the PCMCIA and harddisk option, the A600 cost more than previous home user models and didn't do more. So it didn't sell well initially, and even less so in the USA who had already looked the other way as I said.
The A1200 and the A3000 before that fared better there.
The A600 isn't region specific, so it can be used with NTSC or PAL, but
actual users of A600s in the USA seem extremely rare.
I type too much
The direct answer is: it was made in Scotland, released internationally and did both NTSC and PAL out of the box. So there isn't an "NTSC" A600 model, only software made to run well on both or "made for PAL" since
the greatness of Amiga was more or less discovered by the more cultured Europe. OK I'll have to stop typing and start ducking