I sold a chrome helmet last year but they are relatively rare but I think the Johnny Cecotto one on the page you included in your posting is probably more rare.
I've seen some of the helmet clocks before that were tough to separate, I simpley tried change of temperature to get them apart. So try from room temperature to the refrigerator for 15 minutes / then if that doesnt work try the opposite ie. from cold to warm. It worked perfectly for me.
Good luck - Darren

: A photo posted by @legendary_seven on Mar 6, 2016 at
: 5:12am PST
:
: Hi, I bought these two vintage Heuer chrome helmet clocks today.
: How rare are these? I've never seen them before until today
: while I was researching them, I usually see the none chrome
: ones.
: The one on the left is in working order is the yellow second hand
: original?
: The one on the right is missing the seconds hand and doesn't seem
: to work.

: I'm looking to probably list both for sale the none working one I
: might look into replacing the movement if it doesn't sell and
: keep

: How do I removed the helmet form the base the right one was easy to
: remove, on left one I didn't want to be rough and end up
: breaking anything.

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: