Well I decided to take the plunge and re-tap my SE heads to use standard modern handles. Initially I was only going to do one as a test but, as it turned out to be quite easy, I went ahead and did them all.
Here are the converted heads above their original handles. From left to right: Gem Damaskeene 1912, Ever Ready 1914, Gem Junior Bar.
Also showing the 10mm 5M stainless steel dome head allen bolt I used to convert them. I decided to go with an allen end bolt as this has no sharp edges that other bolts have.
I drilled the original hold in the head to 4mm and used a M5 x 0.8 tap to re-thread the head. I then screwed the bolt from the inside of the head. This was easy for the Junior bar as there is bags of room but for the others was a really tight squeeze as there isn't enough room to fit the bolt behind the threaded hole. With a little pulling I could bend the metal a few millimeters to allow the bolt room and easy enough to bend it back afterwards.
If anyone is planning on doing the same I'd recommend a slightly shorter bolt, maybe 8mm, which would have made it a lot easier.
Here's a close-up of the Junior Bar bolt attached.
and fitted to a Cadet handle.
All new handles fitted: Weber Bulldog, ATT Kronos and a Cadet