I wonder if this received a TOPS number for the main line filming work done in early 2023, alongside three other Bluebell coaches (1464, 1482, 1818).
So are you saying that 1818 would have operated on the mainline a few years ago under its original number, 1818, whereas the earlier coaches would have received a different TOPS, as seen by 2526?
1818 is currently registered on TOPS under its original number as 1818. The Bulleid coaches (such as
1464, withdrawn in 1968) did not survive in use long enough to be given TOPS numbers so it makes more sense to use the 99xxx series in TOPS originally used for "privately owned coaches."
The 99xxx series was used to distinguish privately owned coaches from BR stock - this quote is from the preamble to the list of 99xxx coach numbers in the Platform 5 combined volume
Locomotives and Coaching Stock 1986
This list comprises privately owned coaches which are "plated" to run on BR. The BR PO number is carried on a yellow plate affixed to the solebar. Coaches have to be passed by BR each year and this is denoted by a white equilateral triangle painted on the solebar with the year painted in black.
After privatisation, a separate number series for privately owned coaches was no longer relevant, and preserved coaches registered to operate on the main line could use their original TOPS numbers (like 1818). This made sense, especially if the coach itself retained its old TOPS number painted on the side. The 99xxx number series wasn't completely abandoned, and it's interesting to see some recent additions to it.
I remember seeing the yellow plates (with the 99xxx TOPS number) fitted to privately owned coaches used on railtours, such as the SRPS set. I think it was fairly common for the painted number on the side of the coach and the 99xxx number on the yellow plate to be different - for example the coach might display its old BR TOPS number despite being renumbered as 99xxx, similar to a preserved loco displaying its original number despite being renumbered to 89xxx in TOPS.
I don't think these yellow plates are now required - for example the Royal Scotsman stock has the TOPS number under the data panel at the end of the coach, and the VSOE is similar.
The Royal Scotsman is mainly formed of 99xxx series coaches, but coaches 315 and 316 (which were put into service in recent years) use their 3-digit Pullman numbers in TOPS.
Some examples from the Royal Scotsman and VSOE where the TOPS number is visible:
99968 "State Car No.5" previously 10541
https://www.flickr.com/photos/tractorclag/27229356310/ (Photo: Roy Bradley)
https://cs.rhrp.org.uk/CarriageSurvey/displaycarriage.php?Ref=ZKRUL2P0AWBE95YOMI6Q
99967 "Dining Car No.1" previously 317
https://www.flickr.com/photos/rob37430/11355085423/ (Photo: Robert Dumelow)
https://cs.rhrp.org.uk/CarriageSurvey/displaycarriage.php?Ref=8SLDWH1VKIZ932JYB5EP
316 "State Car No.1" previously in departmental use as DB 975608
https://www.flickr.com/photos/67444577@N02/54861693672/ (Photo: Andy Cole)
https://cs.rhrp.org.uk/CarriageSurvey/displaycarriage.php?Ref=N9730USQFTKLOC8MJP1R
99543 "Vera" previously 284
https://www.flickr.com/photos/nigelmenzies/12018037295/ (Photo: Nigel Menzies)
https://cs.rhrp.org.uk/CarriageSurvey/displaycarriage.php?Ref=GN053SMWQ7LKDZF68OAP
The VSOE is interesting as the named Pullman coaches use 99xxx numbers in TOPS, but the 1996 Platform 5 combined volume suggests they may have reverted to their 3-digit Pullman numbers in TOPS for a few years after privatisation (they are listed under their 99xxx numbers again by the 2000 edition).
An oddity in the 1986 combined volume is that there are a handful of surviving CCTs (S1439, S2010, S2239, S2516) and PMVs (S1712 and S1865) in the Southern Region number series, which appear to have retained their original numbers in TOPS.
I mention this because there are two Southern vehicles numbered 1464 - the Bulleid coach mentioned already (withdrawn before TOPS was implemented) and also a PMV -
https://cs.rhrp.org.uk/CarriageSurvey/displaycarriage.php?Ref=MPBCKW1NORF9J3G50IYD
This is perhaps one reason why the Bulleid coach 1464 was likely given an (unknown) 99xxx series number - to avoid using a number series in TOPS that had
previously been used for PMVs.
Which is a good excuse to link a couple of excellent photos of PMVs in use during the TOPS era:
PMV on the back of a Salisbury - Exeter train in 1978:
https://www.flickr.com/photos/pics-by-john/21023245814/ (Photo: John Woolley)
A couple of PMVs in the consist of a parcels train passing Beeston in 1979:
https://www.flickr.com/photos/pics-by-john/8412452547/ (Photo: John Woolley)