Schneider Symmar 5.6 / 240mm on Sinar DB board

[NormaN]

Hello

I have a question about the Schneider Symmar 5.6 / 240mm on Sinar DB board

Has anyone had any experience of dismantling/revising the spring mechanism for the aperture ring? It's stiff.
On the back, the fastenings are supposedly sealed: how do you remove this seal and how do you reseal it?

Thank you for your help!

NormaN