Picked up an old Rotex labelmaker:

The non-working one - looks identical to my other one from above

I have another one which looks almost identical, however it lacks this white pin which you can see on the front of the new one: <EDIT> Working one has very, very dark grey plastic. Faulty one has black plastic.

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Note two-position white pin

I'm wondering whether the pin is used to change between 3/8" and 1/2" labels?

<EDIT> In the cold light of day I can now see markings next to it </EDIT>

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Sized appropriately

My big question is to try and find out why it doesn't advance the tape when I half-press the handle or 'ka-chonk' down on a character.

Possible issues - I'm using cheap 3rd party tapes from amazon, however they work perfectly in my other one. That's all I can think of, but then I don't know these things very well. They pre-date me by a few years, and the one I had as a kid was plastic.

Here is my working labeler:

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This one works

A user manual would be extremely helpful in working out what the button does, but maybe someone knows the answer?

Either way, I'm hoping to find someone who can help me work out whether it's busted or it's a user error?

Thanks in advance for any help!