Without access to factory records it is impossible to accurately date the Senior.
All Senior air pistols from the first straight grip model onward had the thumb operated stirrup barrel catch. Obtaining a replacement could prove difficult. Chambers or Knibbs may be able to help, at a price.
The Mk.1 air pistol had the sliding top breech catch.
Elbow grease plus various grades of wet and dry is the best way of removing rust and pitting.