your Bias depends on your plate voltage ..... and then where do you want to run them .....
I always go with 70 % ....
can't go wrong with this simple vid from Eurotubes
it looks like someone had put a resistor over at least one of those .... most likely to bring the voltage where they could bias EL34's or vice a versa .... if it was originally running EL34's .... they did it to bias the 6550's
GC has one ....
https://www.guitarcenter.com/Used/MESA-Boogie/Used-MESA-Boogie-Widebody-1x12-90W-Guitar-Cabinet.gc?algoliaQueryID=afb52c34e2057d873fe6e46bb104e158&algoliaIndexName=guitarcenter