I recently acquired an antique Koken barber chair. I assume that the number stamped on the thin metal plate with the hydraulic fluid fill cap (seemingly rusted shut) under the seat is a serial number. The number in question is PCP1898. Can anyone determine from this number when this chair was manufactured? I can post photos if that would help.

It seems clear that I will need to add hydraulic fluid to make the chair function better again. Can anyone tell me what kind of hydraulic fluid to use, and to what level to fill it.

Thanks in advance.