The plug shape/style isn't that important. I can switch out the plug. I just need to figure out the input voltage of the circuit. It looks to be powered by 2 rechargeable AA batteries, but that's all I know so far.
To recharge 2 AA batteries, you need more than just 3 volts. But, I can't verify it is 2 AA batteries in it, but I can't just use AA batteries since the 2 batteries are soldered in, hence being rechargeable. I need to open it back up and just figure the circuit out.
Judging by its apparent age (made in 1960s-70s): If it has 2 rechargeable cells they would be Ni-Cads @ 1.2volts each = 2.4 volts. A "smart charger" that can charge a 2.4 volt pack should be able to charge it, assuming the cells are still OK which is probably not the case as it's an oldie flash. So, I would replace the old cells with new ones before attempting to charge it.
It's a lot of work though for a flash when you can buy another working flash for peanuts.
If you do open up the flash be careful. The capacitor inside a flash can give you a nasty zap of hundreds of volts.
$50+ for another flash is quite a bit of money when you're on a limited income. And I'm not too worried about opening it up. I've done vintage electronics repair for nearly a decade now. The cap in a flash is by no means the highest voltage I've been shocked by. I'm not so much worried about repairing the flash, that's the easy part. Figuring the proper input voltage is the important part so the coil inside the flash doesn't get fried.
I saw a better, nicer Minolta brand flash much like that at Goodwill for $7 last month. That is about the going pawnshop price for "tall" or "cigarette pack" style, 2-AA cell flashes these days. I would definitely look around at low-end stores like Goodwill, or yard and garage sales, for this style of flash unit. There's no need to spend anywhere near $50 for this kind of flash; and if it does cost $50 for this kinda' flash unit, it should come with a working, clean, Minolta HiMatic or Canonet camera as well! Legacy and orphan/generic/store-branded flash units like this are not bringing much money these days.
A 3V charger would work for 2 NiCad batteries. You will get 3V to charge the 2.4V battery supply and that is about right. You always have a little higher charge voltage than the battery voltage to get a full charge inn the batteries. You will also have to figure out the proper polarity of the charger for the plug on the flash. If you check out some of the resale stores you should find something there. I have a couple of the Honeywell 770 flash uits that have removable power packs and I repowerred a couple of the packs with 2600 mah nicads. They do recycle at full speed for a longer time than the original 1500 mah batteries. I haven't been able to run one pack low yet.
I have a couple of similar flashes that I picked up at thrift store prices; one's a Braun that takes batteries and the other is rechargeable (and that one I haven't even tried using yet). The instructions with the rechargeable flash which is a Bauer E 160 Ultrablitz indicate it has a 4.8 V NC accumulator if that's any help. I don't know just because it's a similar style flash if the voltage would be the same.
Keystone mostly made inexpensive plastic cameras that often had built in flashes; I have one that's for polaroid style film. I think they were sold mostly maybe at K Mart or Sears. Apparently prior to that the company had made movie cameras. I don't think you'd find a lot online about your flash since Keystone cameras don't seem to be particularly collectible.
I have the same flash for my Yashica Minister D. The cord to charge it is simply a 120 volt AC cord that my Remmington Razor uses. NO transformer! 120 volts directly into the flash unit. The charge circuit inside is simply a capacitor to limit current and a diode to convert to DC. The charger is a really basic 25ma constant current charger with no regulation. If you replace the batteries, (very dead by now) ONLY use 600Mah NiCads. Larger cells won't charge and NiMh will be destroyed in no time.