Here are the original Storno service manuals for the radios.
They don’t provide any information about the VHF/UHF conversion process obviously, but they do have some useful information about tuning up the radios, as well as circuit diagrams and fault-finding information.
VHF Storno manual
UHF Storno manual
Here’s GWM radio’s original conversion docs for 2m. Note that their instructions are somewhat different to mine and document using a diode matrix board.