Was reading through the encyclopedia and it states for 1915 steering arms, similar to 1914 but right side no longer has hole for speedometer mount. Did all 14's have that hole in the arms or only the ones with speedometers? I would think only the ones with speedometers had the arms with the hole.
Sounds about right, parts book lists left and right spindle arm, right with a speedo mount hole.
Then just the one for both sides by the end of 1916.
I can not imagine that Ford production book keeping was so exact that the arm with the hole stopped exactly with the last speedometer! Seems reasonable that speedometers would cease, then after a run of production having arms with holes but no speedometer drive in them, Ford would use the same plain arm on both sides. Just how long and when that transition took place would be the real question.
Speedometers were standard for all Model T's in 1915 model year. The 1915 model year ended July 31, 1915. Speedometer drilled RH steering arms have been seen on seemingly original cars built long after that date.
If you are trying to build an authentic 1915 Model T it needs the entire speedometer installation including the steering arm.
The speedometer suppliers couldn't keep up with Fords speeding up of production, so many 1914 cars were delivered without speedometers - at a $6 reduced price according to a letter dated November 4, 1913 cited in Bruce's encyclopedia.
True enough, supply was a problem in 1914. This was not the case in 1915.
The speedometers in 1915 (and mid-late '14) were the Ford Special, all of which used the same swivel mount that required the hole in the spindle arm. Therefore I would say that the arm was used continually from its introduction through the remainder of the factory-supplied speedometer era, which was the autumn of 1915 (1916 model year).