I don't have any records of when Los Pinos Tanks was cut down from a standard 50K gallon tank to the size it is now. If you look at the timbering on the back side of the tank you can see where they cut the back 1/4 off.
According to Ticket To Toltec, the tank was rebuilt in 1986. The reconstruction would have consisted of repairing or replacing the big under frame timbers and installing a new redwood tank. As I understand, the new tank and roof were off the shelf items from some wooden tank company and no attempt was made to replicate the original one-of-a-kind tank, which is really too bad.
As far as it not fitting the 315, that is probably because 315's tank is too short (as in height) to get the spout to line up with the manhole. It seems to work fine with the taller tenders.