The RV12P is a German valve that is complete with holder. The base is six pin. The valve is an RF pentode, it has a 12.6 volt heater at 0.07 Amps and that the anode voltage is 250 Volts.
The attached data sheet is for the RV12P2000 which is probably the correct designation for this valve. Also we have a second data-sheet here.
This image shows a similar valve built into a holder.
The underside view shows the all glass valve with leads passing into pins that are held in spring contacts. Data-sheets show that this series of valves had handles on the base. This image seems to show where a plastic component has become detatched.
The top cap fitted into its connection.
The wide glass tube envelope is 20 mm in diameter and overall is 47 mm tall.