See below links to manuals, etc. for legacy DH Type 5000 Pressure Balances.
5201, 5202, 5203 gas operated, liquid lubricated to 15000 psi (100 MPa)
5301, 5302, 5303, 5304, 5306, 5316 oil operated to 72500 psi (500 MPa)
50316 Automatic oil operated to 75000 psi
5501, 5502, 5503 gas operated, liquid lubricated for differential pressures at elevated line pressures
See this article for Type 5300 replacement o-rings,
DH Type 5300 replacement o-rings
If you're using a Type 5000 Pressure Balance with software to solve the pressure equation, the value to use for piston assembly density is 7920 kg/m3. Some calibration certificates show the reference but not the value, "The mass of the piston and plate assembly is adjusted so that under standard conditions the as left apparent mass of the assembly is equal to the apparent mass of a 0.2 kg AISI 304L stainless steel laboratory reference mass ±0.001%."