What does the header of a "bridge group"-package look like?

What does the header of a bridge group look like?

https://i.imgur.com/8f41Ofq.png If you look at my diagram.

Router A and Router C is communicating over tagged vlan 100.

And Router B and ISP-Cloud is communicating over vlan 50.

How is the package header looking between Router B and ISP-Cloud?

If the bridge-interfaces instead was a dot1q tunnel-interfaces it would look like this https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjM1yLFsg8v_y0hwsI5T-VaK1Ex4d0ZE0jDzjAkDopcINmTXIrf4Ti2d2VDAOaPVAH4pPCfgg-eZJptQlFZN0PWxaM3z5R8B6OSHu-WBEwmMF94nsHPZ4Sqf7zFkpGlWYn2pni9R-5uXjM/s1600/qinq.gif an inner and an outer vlan in the header. 50 would've been the outer, and 100 would've been the inner.

With bridge group on the other hand, I do not know how the header looks. I just know that vlan 100 ends up at Router C and if I changed it to e.g. vlan 200 on both ends it would also work.

So anyone know how the bridge header frame would look like in this scenario? Google haven't helped me.