Back in 1920-1922 did they generally use STD black iron pipe on boilers, or did they use XS or XXS?

Standard sch. 40 black steel pipe.

anybody able to post a link to the relevant mechanical code?

Dan Hollihan MIGHT have a 1922 code book, but I doubt if anyone else does.

Does it exist in any code books today? There is very little protection for the homeowner with regards to their heating system, because it doesn't affect the rest of the population. Some codes cover burners and venting, and any connection to the public water supply. But, the piping for the system is usually not even looked at. Dan may have some old design books, but even those don't guarantee that a certain pipe was used by a plumber trying to replicate a design.

not a true statement in massachusetts.
we are required to file for a "mechanical" permit. it is required for ductwork and hydronic piping whether it is new or remodel.
so when i do a new house or an addition or a bath remodel, if i touch the heat.. it requires a 'mechanical'permit that is not covered under the regular plumbing permit.
here is one from cambridge mass.... [www.cambridgema.gov]

Yes.....Mass. does collect money for those. My statement stands, with the exception of the boilers done by packy.