VHF output uses a different type modulator: the one in my TS1000 reads "UM 1082 LA2/3" on the label. Versus UM1233 in the UHF output ZX81's. And these modulator types happen to have their RF output at a different location.
I'd assume that it was easier/cheaper for Sinclair (or Timex) to provide for cutouts in different locations in the case, than have custom RF modulators produced. In terms of injection molding a case like this, it is done using some insert(s) that are placed or removed in the injection mold before producing a batch. Not sure if the same method was used for the Sinclair logo on top vs. the recess where "Timex Sinclair 1000" sits on the TS1000's. Surrounding area seems too smooth - maybe there were 2 different injection molds used for the upper half of the case?
Anyway I'd guess your US model ZX81 looks pretty much like a TS1000 inside. Maybe it was an early production model before Timex-specific customizations were done?
https://www.ebay.com/itm/Astec-UM1233-U ... Swh6xTtqov
VHF has the jack on the right side of the modulator case.
https://tr4.cbsistatic.com/hub/i/2011/0 ... 00x800.jpg
I suspect this is to prevent mistakes in assembly. The location seems to make a "key".