Feral,
1. the Colt original t/b spring is too heavy and not needed. It beats up the cylinder. I prefer the medium tension Wolff spring, and it's good quality.
2. the washers are only little red nylon things that tear apart and became worthless. None of my 2nd gen Colts have them but I've never had a 357 so maybe they were unique to 2nd gen 357s. Early 3rd gens have them but they were soon abandoned by Colt.
3. the "LAP xxxx" is likely a local law enforcement serial number stamped when the gun was found missing it's serial # on the frame. I had one stamped similarly by the local sheriffs dept: FCSO XXXXX (Fresno County Sheriffs Office). They too stamped it on the butt which is not properly legal. The main frame (not the grip frame) is supposed to have the official serial #. I wouldn't be concerned however, it's obviously a LE agency re-numbering.
Original serial numbers can be retrieved, see this thread on the S&W forum, post #33:
http://smith-wessonforum.com/s-w-hand-ejectors-1896-1961/238409-455-picture-thread.html
4. the number on the loading gate is a factory assembly process # and will match the number on the bottom of the frame covered by the trigger guard before 1908 and after that, on the back right side of frame visible with the right grip off until 1940. This will help you date the gun to pre or post 1908.
5. Looks like a round trigger guard which dates it to between 1894 and 1917. So with the assembly # location above, that will get you in the correct 10 year period.
If you can still see the "Verified Proof" stamp, a "V" with a "P" in it, on the left front trigger guard bow, it was made after 1902.
It will have a "V" notch rear sight and tapered front sight blade in either period. Since most of your stampings detail is gone, they're no help in dating it. 32-20s were made thru out that period but cyl and barrel may have been changed anyway.
6. Definitely worth more than any Vaquero.