Most accounts of the construction of No 21 indicate she was constructed by using parts from NPC No 5, The Bodega. The NPC took delivery on No 5 in 1875. The Bodega was one of nine Baldwin 4-4-0s purchased by the NPC from Baldwin between 1874 and 1877. With the exception of No 9, the M S Latham all were virtually identical with 42 inch drivers, 12×16 inch cylinders, and a weight 0f 44,000 pounds. What set No 9 apart was detailing as it was President Latham’s engine. But mechanically all nine were virtually the same. Bill Thomas believed in purchasing stock generic Baldwins because of parts availability and interchangibility.
No 5 was dismantled sometime around 1897. When the time came to build No 21, Thomas had a complete drive train available. The only photo I’ve seen of Number 5 is a partial engine shot.
This shot shows the rear half of the engine and the tender. It was taken in 1892.
Here’s a shot of No 3, the Tomales. Note the similarity of the drive train to this shot of No 21 when in testing.
If you look from the boiler down, the drive train, is virtually identical. The NPC ran both kinds of pilots shown in these photos. It is said that Thomas did some drive line upgrades as part of his rebuild, installing his patented American Balance Valve and adapting the drive train for the switch to a cab forward locomotive.