At least for the TS1000, RF (TV) output is on V
HF channel 2 or 3. Versus U
HF channel 36 on a regular (UK) ZX81. Also note the channel select switch on the bottom, which is not present on a UK ZX81. And the somewhat-conductive silvery paint + metal strip inside the case on US models (for EMI shielding).
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?