I AM stupid..... should have seen this ages ago.
Of course 243.0 is exactly two times 121.5 !
Any technical reason that anybody knows of, like running off the same crystal?
If one wanted to be pedantic, 243MHz isn't UHF, because 'formally' UHF doesn't start until 300MHz. But the answer is of course simply, that in aviation "118-137" is VHF and "225-400" is UHF.