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I am junking out a barn prior to demolition, I have come across a
fullish calor bottle about the 19kg size but it has the old female
******* thread. Are these things likely to be collected by museums?
Any views as to how to make use of the propane? It's a dull
gray/khaki.

If it is grey I think you will find that that Propane might be Butane. :-)

The blue colour for butane containers is relatively recent but I am not
old enough to remember Calor Gas with female threads. Grey and male
lefthand threads were around on the standard size bottles in the 50s
through to at least the 70s and the lh thread is still in use on the
small (12lb empty) containers.

--
Roger