Hi Tom,
That's a really interesting observation -- a little detail I had never noticed before. Thank you for posing this question. I think you're probably correct that one of 15's tender trucks was given to 16, but I'm not sure about the years.
The first volume of Jerry DeVos' series on "The Sandy River & Rangeley Lakes Railroad and Predecessors" has a detailed history of each of the SR&RL Baldwin engines. DeVos makes no reference to a truck swap, but he does note that while there are no known photos of 15 in service later than 1923, it doesn't appear to have been officially retired until 1929. Furthermore, he shows a couple of photographs of 16 in Rangeley in September, 1928 (well after its rebuilding from the 1924 "Roundabout" collision with 23) with two matching tender trucks. So assuming these dates are correct, I would guess that the truck swap happened sometime after 1929.
Of course, that just presents the new question of why the truck had to replaced in the first place if it wasn't as a result of the 1924 wreck.
Something else I'm noticing now as I pore over old photos is that 16 and 18 appear to have had different tender trucks! The trucks on 18 were more like those on 15 than 16. I have always been in the lazy habit of thinking that 16 and 18 were identical, both as delivered from Baldwin and as rebuilt by the MeC, but that really puts this assumption to rest. (The preserved tender tanks of 16 and 18 in Phillips already show that they had slightly different designs as far as the tank itself was concerned.)
-Philip Marshall