I have Braun Hobby EF300 that looks well thought out. It dates to 1968 in its manufacturing marks inside the case.

The coiled rubber cord fell to flakes when I tried to extend it after it was gifted to me, unused for about 25 years, after a few decades of being well used.

It certainly does not look like a prototype or even a second generation unit. So I would easily put 'battery powered condensor based' units to the early 60's. Not that they were mass market. They had to put out a big whomp to be useful and supplant the light output of flashbulbs of the day. An M2 or M3 is a lot of light in a small package.

The best resource to answer the question would likely be the adverts section in the front of the older BJP annuals.

You certainly know you are using this thing after the pack hangs on your shoulder for a while. But you do get a guide number of 60 in metres, or 30 at half power, and an option for wide or narrow beam spread.

My EF300 still works well after replacing the pack to head lead, and sourcing a replacement for the oem 2 of 4V each wet cell batteries. (I went with Hawker Cyclon 2V 5Ah AGM lead acid cells, and built a case under the original pack to house them).

Yes, I carefully allowed the capacitor to reform on reduced voltage for quite awhile before trying to discharge it.

I still use this pack for theatre show shots, during dress rehearsals with the head mounted on a light stand, as one of the usually 4 flashes I use to light the stage.
I have fitted it with a newer battery bank - 2 6V 10AH and 2 2V 6 Ah. Heavy- but it keeps the stand stable when hung as a sandbag substitute.

The thing fires over 160 shots in a night on full power, and still recycles in under 8 seconds by the end of the night. I fire it by a Wein strobe through. The other three run off Cactus radio strobes, but the EF300 has a synch voltage approaching 600V on full power.