Shot Practice Tracer
Ammo Rob,
As several people have indicated earlier, your projectile has excessive corrosion which would indicate that it is not AP, but soft material and therefore a Shot Practice Tracer.
Ref: Regulations for Army Ordnance Services Part VIII Pamphlet No. 3 - Marking on Land Service Projectiles for Guns, Howitzers and Mortars.
The following information is indicated by stamping
a. Calibre and numeral
b. Manufacturers initials (or recognized trade mark) and date of manufacture
c. Lot No. of empty projectile
d. G or HR or G&HR - denoting the use of the projectile for gun or howitzer, or both
e. H or L denoting Heavy or Light
f. C.S., F.S., or B.S., denoting that the shell is made of cast steel, or forged steel, or bored from the bar
Ref: Regulations for Army Ordnance Services Vol 4 Ammunition Pamphlet No.7 QF Separate Ammunition Pt 2 QF 25PR GUN/HOW
Shot Practice/T
Shot Practice/T are designed to give the same ballistics as AP/T Shot but are normally made of cast iron. Shot, Practice/T Mk 1 is the only exception, being made of mild steel.
The Mks 1 and 2 Shot are fitted with Internal Tracer No. 2 Mk 7, all other shot being recessed to receive an integral tracer filling which may produce a red or white trace according to the M of F design.
The tables list 5 projectiles:
Shot Practice T Mk 1, made of mild steel with tracer shell No. 2 Mk 7
Shot Practice T Mk 2, made of cast iron with tracer shell No. 2 Mk 7
Shot Practice T Mk 3, made of cast iron, recessed base to accommodate a steel closing plate, shellacked paper disc and celluloid closing cup
Shot Practice T Mk 5, made of cast iron similar to Mk 3 but has a modified tracer cavity
Shot Practice T Mk 8, differs from the Mk 5 in size of tracer cavity.
Based on this information I am assuming your projectile is a 25 Pdr Shot Practice T Mk 1 or Mk 2. I base this on the photo of the base as it looks like there is a lighter metal in the tracer cavity, which would be the tracer shell. The B S on either side of the base indicates bored from the bar, and C-S 55 12 would indicate manufacturers monogram and date of manufacture.
Do you still have access to the projectile? If so, can you clean up the base and see if there are additional markings and if it is actually a tracer shell screwed into the base.
If you are getting rid of the projectile, you can probably guess where you can send it to for it to recieve a good home.