The entry in the Parker serialization book, Price & Fjestad reads for 21005
PT U 0 S 10 32
Grade 0 gun with plain twist barrels.
Under lifter opened action.
No Optional extras.
Straight grip.
10 bore.
32 inch barrel length.
Judging by the number of similar entries around this one in the serialization book, it was a very popular configuration.
Pictures can get you specific commentary.
See page 121.
https://www.amazon.com/Parker-Identi.../dp/1886768374
If you care to really learn and want to study these guns, just go ahead and get the book as soon as you can. If you don’t get it now, you’ll get it later and wish you had it before.
Parker started production of the Grade 0 (zero) in 1869. Parker started its grade numbering system at zero to denote their lowest grade made at that time. As Parker introduced the named grades they named Grade 0 guns R, S, T, U and VH. Except for the VH which describes all Grade 0 hammerless Parkers, the other names describe Grade 0 hammer guns based on various gauge and stock configurations. This grade had Decarbonized Steel barrels in the beginning and later used Plain Twist, Stub Twist or Plain Steel barrels. The early grade 0 guns were back action lifter guns and evolved into the front-action lifter.