Financial calculators puzzle

[Chasfield]

There is an older single line Casio FC-100, which also has the 20+30 button layout. However, the function allocation is completely different (not just swapped around a bit) and there are no unit conversion functions. Older models from other manufacturers (Citizen and Sharp) have no resemblance at all. It is hard to believe that a fully function financial calculator for 5 GBP isn't a clone of something first made by somebody else.

I have ordered one up to compare with my HP12C.