Does linseed oil come boiled?  Or is the boiling something you do at home?  I've never ran into it before.  

I've actually used the Sadowsky everyday cleaning with a very soft cloth that doesn't shed... like an old tshirt, and had great success at maintaining cleanliness under the strings.  

I'm interested to try the linseed oil, though.  

Linseed oil comes as either raw linseed oil (not boiled) or boiled linseed oil. Linseed oil is flamable.
You can buy a bottle at Home Depot and get a lifetime supply.
Be sure to wipe the excess oil off the board as the oil can get gummy.
As far as scraping dirt off the board, old credit or bank cards do a good job with no marks.
 

Is there any information on the various bridges used over the years? From BadAss(?) to Schaller to Hipshot to the most recent?

Considering the mod for my Metro RV5 as well, but now I'm curious if it came stock.  Is there a s/n cutoff, or I suppose the more applicable/selfish question:  would a Sadowsky wiz be able to confirm/deny whether M4146 has the 500k pot?  I'm not concerned about the minimal cost of the part, but rather the effort involved in drilling the hole to accommodate it.  Thanks in advance.

The pot value may be stamped on the back of the pot. It's easy enough to take off the cover plate and check. You can also test it with a DMM if you have one around.

Lastly, why would you have to drill anything? Is it not as simple as "de-soldering" (if that is a word) the 250K and installing the 500K in its place? other than resistance value, the pots should be identical, no?

The 500K pot in my Metro is a mini pot. There's no drilling if replacing with a 250K mini pot but a full size pot's bushing won't fit through the hole.

My new metro mv4 has a 500K volume pot.

TC 450 plus a Berg NV215.