I think you'll find that in the frequencies used by the Pronto it is the other way around.
X-10 was using 310MHz from their first USA RF remote. That was long before there was a Pronto. I think they also used 418MHz here for the Powermid long before others copied that.
And even though they originated in Scotland, X-10 moved to the USA before they started maerketing anything because they saw the market here was much larger than the UK market. They started marketing in the UK and Europe much later.
FCC Part 15 Rules cover 300-1000MHz in the USA. You can use any frequency in that range for things like this. CE did not even exist when the FCC made most of these frequency allocations. CE mandates 433.92MHz for things like this.
The UK used 418MHz for X-10 RF until 2-3 years ago when they moved to 433.92MHz to comply with CE rules.
The first TV remote controls were ultrasonic. Then came IR. RF extenders came even later.
