Second O Ring on cylinders in Fuel Injection Pump ?

[dario]

Hello,
I recently started re-sealing my spare fuel injection pump.
After cylinder has been removed, I noticed that apart from the "regular" O-ring there is likely another one, just at the head.

Is this really oring sitting in the groove ? (picture)
Should it also be replaced ? (I suspect that it seals oil supply area from the fuel supply part inside the housing but outside the cylinder)
Does anyone of the our forum's experts know the correct size of that sealant?

The pump type is PES6KL 70A120 R18z.
Thank you in advance for any information.

Dariusz