Impossible to answer without a DETAILED description of ALL markings.
Depending on EXACTLY what you have, a few hundred to tens of thousands.
are you sure it's not BP for black powder ??? it means barrel bushing, the pistol is a transition between colt 1911 commercial and series 70. your pistol was probably made in 1969.
read what is on the barrel. it will tell you who made...................
Proofhouse.com has Colt sn data.
Colt has never made a 45/410 pistol
AFAIK, Colt never made a single shot pistol
Impossible to answer without knowing who made it.
You will have to call Colt to find out.
made by colt in 1918................
If you call Colt they will tell you when it was made.
Kits were made by Connecticut Valley Arms Otherwise known as CVA
1974