By the looks that is a pair of 18650’s which be recycled batteries from China which can be hit and miss.
18mm diameter, 65mm long, two of with nominal voltage of 3.7v each together get you your 7.4v.
Strip that checker wrap off you’ll see that is what they are. Measure the dimensions up and search for a replacement Turnigy or something with the same overall dimensions.
Maybe something like this Turnigy Nano-Tech 300mAh 2S 45C Lipo Pack w/XT30 | HobbyKing
I know it’s out of stock but the sort of thing be looking for if I was you after checking overall dimensions see if will fit.
Hope that helps. Cheers
Well the bigger question is why only this particular battery works? Like why doesn’t any other 7.4v work?
Buggered if I know. Plug not connecting properly. 
Just a quick update, was having a bit of a look and was able to pry the shell open gently enough to reveal metal pins holding it together. Was worried it was just plastic pins that would snap the moment you touched them.
Will try and open it completely tomorrow.
Ok, so it took nearly an hour of gently prying around each post to get the shell open, not before finding one hidden under the charging handle.
The gearbox looks to be that polycarbonate stuff, though it’s pretty thick in most places.
Motor is a standard short shaft with a metal pinion. Gears at first look to be standard but the AR latch has an extra post, and the sector gear seems a bit bigger, didn’t have a spare set handy to test fit, gears were also bone dry and has some surface rust.
Piston is proprietary as expected but runs a standard o-ring. The piston also only engages with one plastic tooth as the pickup but after that the gears engage with a metal plate that free floats about the bolt carrier, not dissimilar to how the old springer shotguns work.
The main spring unfortunately isn’t a standard size, thou is pretty hefty.
Micro switch on the side engages with dust cover for what I assume is to to stop it from running without being chambered.
Screws are all self tapped.
No FCU so I still can’t understand why it won’t cycle on anything but the stock battery.
There send to be a good amount of detail put into making it not cumbersome to pull apart, like springs, bar 1, being captive, the entire gearbox comes out in one piece and barrel assembly just free floats so it’s all easy to take out.
I put something back wrong thou so I gotta pull it apart again to fix that 
.
Just some additional pics
Very interesting, a lot different from the norm.
All I can think of with the battery is the plug may be non standard and not connecting properly. There are many variations of the JST plug type which it looks like it’s from that family from the picture. Which one it is being the hard part to find out.
I’d be tempted to deplug it and put your common plug type on.
It’s mini Tamiya plug. The only other thing I can think of is the motor but it looks like a generic Gen8/V3.
Polarity the same? Some weird voltage perhaps
Thats some beefy wiring and a hefty spring maybe it uses a higher than 2s or 3s battery
That IS different… 
I guess something as unusual as an AA12 is going to be full of proprietry stuff.
I’ll have a look at the polarity when I get home but it’s only a 7.4v battery.
Although in saying that when I pulled it apart the wires were on the correct poles on the motor.
Strange that one, can’t think of anything else.
Interesting bit of kit inside. Thanks for taking the time to post it.
There really is nothing to indicate why it won’t run on anything but the supplied battery. Be worth checking if the wires are positive and negative like the colour would suggest, and checking the rotation direction of the motor on stock battery vs your others. Wouldn’t be the first time I have seen a blaster with “wrong” coloured wires.
So it does turn out the battery is reversed polarity.
Top is a standard aeg battery
That is either the dumbest or smartest decision made…
Aha had to be, not much else to it. Likely easy solution to motor being upside down miss jane.
Will have to make a plug adapter if I want to use any other batteries then
So are they selling these AA12s with dedicted reverse wired batteries or did someone at the factory have a brain fart? 
An adapter’s your simplest solution, but that blaster would seriously trigger my OCD and I’d have to rewire it to run on normal polarity… but that’s just me. 
Unless they used the wrong colored wire on the battery it seems it is reversed, the wires are attached to the matching post on the motor, and it would explain why I would only get a clocking sound from it when using other batteries.
I don’t see why they didn’t just plug the motor in reversed instead of making other batteries unusable, it’s not like they sell additional batteries for this.
Tamiya plug wired like remote controlled car spec
I’d redo the plug on the blaster. Swap the blaster plug polarity to suit your batteries. Or motor, either one.