I brought a batch around 4 years ago with a external thermostate
Aqua one was I belive the manufactura.
I found them about as reliable as a busted clock.
Having said that I have not been impressed with Aqua one at all
The idea of stainless heaters is a product of the changes in the law in the UK in the 1970,s.
For those of you who don't remember the way heaters were sold and used in that period, the heater came as a separate unit and the stat as a separate unit you cut the live or neutral on the stat and wired the heater across it.
Yes honest just twist the wires and a bit of electrical tape .
Naturally the legislation changed and went over the top as always. before you used a test tube to encase your heater. So triple glass, pyrex, and laminated glass, were all requirements, thus the idea so a stainless heater cover was to overcome the extra laws regarding the glass covers for heaters.
The sad fact is that the separate heater /stat worked better as you placed the stat in the upright position and the heater as far away from the stat as you could. thus getting a better spread of heat.
Oh the joys of metal framed tanks and air driven algard U/G filters.
Not forgetting seeing the first spotted plecos.
A time when Rena pumps were hi tec and Tetra were the only food manufactures apart from "Super Ants Eggs"
Pip Pip
Bob