Found this. The 6 is punched over another number but I'm not sure what number. It runs from the right top tip of the 6 down to the loop of the six. It is curved but doesn't seem to match the curve of the 3.
Your guess is as good as mine! Colombia has some really wild repunched dates. It's anybody's guess, but that could easily be another 6. Here's a 1966 1c I found:
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