Bob,
The 12XXX range .455 TL does not fit in either the 1st or 2nd version 455 TL serial ranges; therefore can only be one of the 291 of the 3rd version at the bottom of this brief:
The 3 Versions of Hand Ejectors chambered in .455 Mk II for the British are:
1. ".44 HE 1st Model" TL, with .455 Mk II chambering: 812* factory reconfigured unassembled or unsold ".44 Spl HE 1st Models", often not .455 stamped, original chamberings unknown, 666 for the British thru #1104-10417, the extra 146 #9858-10007 for the commercial market; 123 in England and 23 in the US [N&J pgs. 204-205]. These 812 .455 TLs were serial #'d in the .44 1st Model serial # range of 1 to 15375. Shipped 1914-16.
* SCSW reports "over 800", but by serial # count, it's actually 812, 146 of which are commercial guns [S&WN&J pgs. 203, 204 & 205].
NOTE: Of the 146 .44 HE 1st Models that were reconfigured/built as .455s assembled some time after the first 666 military .44 1st Model .455 TLs and sold commercially, 123 were sold to the British, shipped to Wilkinson Sword 10/1/14 and 23 sold in the US, shipped to Shapleigh Hardware in St. Louis, MO. shipped on 1/1/1918.
The 23 at some point were converted to .45 Colt and it's unknown if by the factory before shipment to Shapleigh or after delivery to Shapleigh. However even IF converted by the factory (as suggested in a September 2013 Rock Island gun auction narrative), the revolvers would not have a star on the butt or a rework date on the grip frame because they did not go back to the factory for conversion as rework.
2. ".455 Mk II HE 1st Model", TL: New .455 British serial # range 1 to #5461 [HofSW pg. 201] made 1914-15. Thus creating a possible c. 68 of the 812 serial #s above duplicated with the .44 1st Model .455s.
3. ".455 Mk II HE 2nd Model": continued in the .455 1st Model British contract serial range beginning c. #5462 to #74755, made 1915-17. Feb 1916, 724 manufactured for the Canadians, chambered in 45 Colt, presumed for the RCMP [HofS&W, pg 203]. Canadians also bought 14,500 2nd Models. And 1105 2nd Models released for commercial sales in the US shipped Dec 1917 to Shapleigh Hardware in St. Louis [S&WN&J pg 216].
"As the Brit contracts were finishing up in [April] 1916, S&W found enough parts to build 691 .455 Triple Locks. These guns will be numbered in the 44 serial number series [HofS&W, pg 203]. I have no idea why they were not just numbered in the .455 series. Perhaps it was .455 barrels and cylinders that the factory found, and they simply turned again to existing 44 frames to use them up. They were sold commercially." Lee Jarrett
NOTE: in the SWCA database there are two entries for .455 TL's with serial numbers 12740 and 12747. Both entries list shipment to Shapleigh HW; Jinks' entry states that 325 units were shipped on 12/29/17.
On that 12/29/17 date, 1105 'over run' guns from the 2nd Model contract sold commercially in the USA, shipped to Shapleigh Hardware in St. Louis. Possibly Roy has identified 325 of those as TLs from the final 691 TLs built at the end of the 2nd contract. That leaves 366 (possibly in the 12000 range) of the 691 that may have shipped to the British military at the end of and as part of the 2nd contract.