The metal-backing was a modification that Roger came up with for the intended Mark V. But he started using it on some of the very-late Mark IVs.
He told me a story about one Synchronar owner who owned a dry-cleaners. He said that the dry-cleaning chemicals that had been absorbed by the owner, were then absorbed into the back of the Synchronar from the owner's skin and damaged the workings of the module. According to him, this was because the lexan (and the gel) were both not as impermeable as their specs claimed.
After hearing that from him, my work-around was to apply a square of black electrical tape to the bottom of my watch's module - a homebrewed solution to the problem. I also decided to never own a dry-cleaners.
For the record, I will state that the Synchronar that I did this to no longer functions - one of the batteries eventually leaked acid throughout the module innards. So I can't be certain whether my "solution" was really a good idea or not...