chemical inside old pressure sensor?

I am looking at an old Pressure Sensor with a dial display (shows Psi & inch hg gauge) on one end and a membrane type transducer on the
other end. On the display there is a label that says "Nr:4" I wonder if this refers to the chemical inside this sealed sensor assembly. Does anyone know what "Nr:4" means? Generally what chemical do they use in these older dial type mechanical pressure sensor?
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Dear Joseph:

Probably not.

Glycerine is pretty common, to dampen mechanical vibrations in the gauge.
David A. Smith
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Mineral oil is also used.
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