SIIa GS didn't have jerry can holders, they had bumperettes, sometimes called cookie cutters.
Only the SIIA FFT had jetty can holders because they had the radios and batteries where the jerry cans were stored in the GS.
I have some SIII jerry can holders, but they will need modification to fit in a SIIA and your tail lamps will obstruct the 20 litre type, if you fit them in the same location as the FFT they will obstruct the opening of your tailgate.
if you look at this image I posted up earlier today you will see what I mean.