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I am going thru my 51's trying to get my collection cleaned up and out. I am having a hard time finding dates for when the the vacumatic 51 went from double jewel to single jewel. Were they made concurrently? Second how common are the vacumatic demi's usually I dont want to keep the smaller pens but these first demi's started to be made just before switch to areometric right? But I do get into the rare or interesting the demi's in the areometric really dont interest me but are the vac version worth saving?
thanks
Alex
