Strange bolt with rubber seal on firewall

[66andBlue]

While I took out the bolt in the square cage nut on the firewall in the passenger footwell to install the bracket for the Becker radio amplifier I was surprised that a round thick rubber disk came along. My 1966 230SL and the 280SL did not have that rubber seal and now I am wondering whether this is perhaps the item #10 in the attached sketch from Becker.
Has anyone seen this part number 10 "Gummidichtung 107 Z 5000-06" and if so, does it look like what is shown in my photo?
Note the hexagonal cut out for the bolt head in the rubber puck.

[goldhamme_rulez]

Since these BECKER spare parts were never part of the Mercedes system, there's little chance of a straight forward reliable answer.

Since BECKER was sold various times to larger automative conglomerates absolutely no historic knowledge or parts remain with the business of today.

Your rubber thingy seems to be doing the inteded job very well though ...

Oliver

[mercakungen]

Mercedes used Karro bolts in sixties. So the bolt is period correct. They used also Kamax, Verbus and NSF.

[66andBlue]

Thank you both for the replies.
However, I am still hoping that someone has seen the part on a car of the same time period. To repeat:
"Has anyone seen this part number 10 "Gummidichtung 107 Z 5000-06" and if so, does it look like what is shown in my photo?"