Speedometer/tachometer seals

[skelteanema]

I had to repair my speedometer as its magnetic rotor seized. I had already bought seal kits. As the speedo was out, I decided to clean up the tacho as well. When dismantling the speedo it had a sealant between the bezel and the glass, and an o-ring between the glass and the slip-ring/face. The tacho only had the sealant between the bezel and glass. The seal kits I have only have the seal for the bezel/glass.

Is there supposed to be an o-ring between the glass and slip-ring/face? Looking at the various sites on Smiths gauges, some seem to show a gasket here where others don't.