Left side right in front of receiver, see related links for diagram and table.
100-400 US Dollars
Depends on condition, from a $100 parts gun to a $1500+ new in the box. Any very good 12c is worth $500-600, jumping quickly nearer to mint. Oh and 1909 is the patent date, other then made after 1909 is has nothing to do with the date of manufacture. For manufacture date if it's after 1920 it will be date code stamped on the barrel see related links for information on the Remington Society of America, if there's no date code need serial to date it.
100-1000 USD
I believe they are date stamped like the rifles on the barrel, see related links for information and look-up table.
from $100 to $400 depending on condition and barrel.
1963
NOWADAYS $100 , NO SCREW IN CHOKES= NO VALUE
That would depend on the caliber of the particular Remington Model XP-100 you have. Remington chambered these hand-guns in a multitude of calibers from .221 Fireball and up to the .350 Remington Magnum, so rifling twists will be different from one caliber to the next and the specific twist in that barrel will play a big role in how accurate a bullet performs.
I think your referring to a model 12 pump action which would have been made in 1913, it doesn't have the model anywhere on it or a date code (to early). The value is greatly (almost completely) determined by condition otherwise it's a 100-1,500 depending. 12-A is a round barrel, 12-B is a 22 short (gallery) round barrel, 12-C is a hexagon barrel, 12-CS is a hexagon but 22 WRF not lr.
100-500
100-600 USD
It's either a 12-C or CS depending on if it's a 22 lr or 22 Remington Special. It's worth between 100 bucks for parts or 1,500 for like new in the box, all depends on condition condition and condition.