Here is the Tech head I'll try with the handle above.
It was listed as a "Ladies razor" but after a bit of research, it seems the only Gillette Tech razors with a head like this and "made in England" stamped below the hole were part of 1940's service sets, so called because they were issued to servicemen during WW2. The "bars" on service set heads are apparently slightly different to other techs too, with an additional lip running along the bottom of each bar.
On this razor the plate on the cap and bars looks to have completely worn through to the brass but it's still a nice looking razor and even though the standard handle looks and feels great, I'm looking forward to cleaning the head up and trying it with the new handle.
Here it is, uncleaned and as it arrived:
Edit: I posted at the same time as the previous post. Yes, the COG of my replacement handle is towards the bottom and I did wonder if I'd be chasing the razor around the bathroom.
Fortunately, with the (solid) tech head on the end, the COG has moved up to around the second gripped area from the top. I'll see how I get on but I want to try the razor with its original handle first, it seems wrong not to.