Tom from SWISCO responded:
October 31, 2017
A stamp of 28D usually corresponds to our 2840 option, while a stamp of 22E would correspond to our 2250 option. It basically goes in the same order as the alphabet: A=10, B=20, C=30, D=40, E=50, and so on. You can also weigh your sash on a scale to be 100% sure.
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