The Mutual Aid companies being unable to connect to the stricken dept's hydrants has many points of origin - the one I heard was NYC in the 1890 - 1900's.

Wish I had a pic of a "titty" hydrant (a hydrant with just (2) 2 1/2" ("Deuce and a half") ports on it.)

The "Steamer" can be one of many different sizes (3 1/2" - 6") and threads - some cities (NYC, for ex.) still use "proprietary" threads. The side ports are 2 1/2" NST - National Standard Thread - so any arriving company can utilize them.

BTW - the "National Standard" of dia., thread pitch and TPI was determined a few years after one of those "y'all come" debacles described here... [Linked Image]