All model 35's have the ball-sear trigger, and a key to tying down the date is the details of the trigger blade (and the dates are just educated guesses on my part). The old models 25D, 27, 35, and 50 used the same trigger mechanism and blade types.
This photo shows the blade used on the pre-war striker-type trigger (top), and a third-generation post-war ball-sear blade. The pre-war blade is case-hardened steel with two adjuster grub screws in the lower face and front shoulder. The post-war blade is solid aluminum with larger countersunk headed screws, both in the lower face, and a ribbed face.
The first-generation post-war trigger is solid aluminum as above, with a smooth blade face. But it has two small screws located similarly to the pre-war ones (even though the trigger internals are quite different). This variant seems rare - perhaps emptying old parts bins? - and just used to maybe 1952.
The second-gen blade keeps the smooth face, but adds the adjuster screws as in the photo. Perhaps up to 1956 on these. The screwdriver access hole in the trigger guard also moved.
The third-gen blade is identical to the second, except adding the ribbed face. Those lasted up about 1962.
This photo shows (top to bottom) the third, fourth, and fifth-gen post-war blades.
The fourth-gen blade is basically a plastic rendition of the previous aluminum. Note only one screw; the previous front locking screw was superfluous as the friction of the adjuster screw against the plastic does the same job. Up to about 1968 on these.
The fifth-gen blade is a complete re-do, stamped blued steel with protruding adjuster screws. Came in about '69 and lasted until the ball-sear guns went out in the early 1980's.