GraphGear 500 cap substitues?

I have acquired a GraphGear 500 with a missing cap. Are there any cheap substitues available? (Except another 500)
Also, the pen has a date code, 5G. My googling didn’t turn up any information that would have helped me decipher. Which date would it correspond to?

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Can’t help on the cap, but the date code is probably July 2015.

The 5 is the last digit of the year (so potentially 2005), and the letter is A to L, representing January to December. Usually the letter is capital, but in a few cases it is lower case.