Union Machine Works was a machining company in Jack London Square that had been "Machining since 1885". A fire on May 26, 1909 destroyed the original building, and a new one was built in short order and the business back up and running. 2 The building fills part of the block on Clay Street between 2nd and 3rd Streets.
A 1905 classified says Union Machine Works was at 3rd and Franklin and a 1921 copy of California Shipbuilder and Metalworker lists the address as 367 - 3rd Street which is near Franklin, and lists them under both 'Steel Ship Repairers' and 'Machinists—Jobbing'. 1
The 1935 directory lists them at 369 - 3rd Street, and William Musladin as president and J. J. Knight as secretary-treasurer. The 1940 directory and the 1967 ad list them at 534 - 2nd Street, so it appears they used both buildings.
c.1948, Henry Charles Wieking worked there as a machinist.
Unknown when it closed, but a SeeClickFix report from 2013 said it was vacant then.