Impossible to answer without knowing all the variables that mean "best" to you.
1915-1916 is a best estimate
1910
Centerfire
150.00
You need the services of a gunmsith.
25-50
In the owner's manual
Not in any reference I have seen
10-90 usd
Proofhouse.com has Colt sn tables.
Maybe 100 or so
Less than 100 USD