Eric Hansmann asks,

How many other lines were rebuilding gondolas into box cars in the 1920s?

Well, not a gon-to-box conversion but similar: in 1929, the B&M rebuilt 100 steel gondolas into their 29000 series pulp racks
with two side doors. I have a bit of a writeup on them here:

http://bostonandmainerailroad.redmansefarm.com/BM29000.htm

Earl Tuson