OK, let's start with your battery pack. I'm pretty sure it doesn't have "blastmaster" on it. No such thing. It would have to be PowerMaster. It has nothing to do with the pack itself, just a decal for the GTAx 1000 PM series. That pack will power all of the GTA/GTI series. The only difference in them is the decal.
Now, let me see if I can give you a rundown on the GTA/GTAx series. The GTA 1000 was the first in the line, being the Ultra model. They came equipped with the "x" coil pin configuration used on the Crossfire Coils. There are only a limited amount of coils available with this pin config. Next came the GTAx 1000 PM (PowerMaster). It supposedly had 20% more power than the 1000 Ultra and used the same Crossfire coil. Some of the first ones had the "x" pin config, but the later ones had the "M" pin config and which is used today on the Ace 150/250/350 series machines as well as the 1250 and the rest of the GTAx series. Finding one of these models would be preferable as it will give you the option of more sizes of coils to use.
Now, when the 1250 came out, it was basically a 1000PM with a few more bells and whistles and the Crossfire II coil. They redesigned the faceplate, added 2 more hunting modes and that was about it. Both are decent coinshooters and have put some nice loot in my pouch. Both are threshold based machines, something I miss on the new Ace series. I still dig them out on occasion, battery them up and let 'em hunt. The 1250 was my first ever detector and will never be sold. It's solid as a rock, never had a problem with the detector itself, although I did have one coil bite the dust.