Aha - if in doubt, open it and read the instructions! I had left the windows all wrapped so as not to risk getting them damaged. After your tip to check they had the protective film on I opened them to find that, yes they have film on both sides, but also that the film on one side gives instructions on cutting!
Also I now realise why it is left and right glass when it is the same shape and flat - they have MSA approval markings on the inside, so making them left & right...
And to think I am always telling my customers to read the manual/instructions first - D'oh!