Though I can’t say 100% sure as it’s never been confirmed to my knowledge and I don’t think WeatherBeeta has info on the Miller’s saddles and serial numbers (I contacted them about my Crosbys and Collegiate some years ago and they said all Miller’s records were lost), I believe the first number or two of the serial number represents the year of manufacture (usually two digits for 2000 and on, and one for 1999 and prior; very early '80s and prior seem to have no number at all). If this starts with an 8, it might be a 1998 model, for example (I believe the Centennial was introduced in 1996, so definitely no older than that). They changed the format of their serial numbers a bit on certain saddles in the early 2000’s, but most saddles had a number that had one or two digits (what I suspect is the year), a space, one digit, a space, then four digits. I’m not sure the rest of the number means anything, as I don’t think many of these had many custom options available and I’m not even 100% sure about the year being the first digit or two, but I noticed a pattern with their serial numbers and noticed that, for example, their XL Excel line of saddles, which were produced from roughly 1997 to 2001/2002 and have this same serial number format, all start with something like XLE7 or XLH00, and I’ve seen a lot of those saddles and never seen anything lower than 7 or higher than 01, so it’s a rough guesstimate that that’s the year of manufacture. I’ve also noticed that the '80s saddles right up to around 1990-ish have brass “Crosby England” nailheads, 1991-ish to 2002-ish (around the time Miller’s went out of business) have black and gold “Miller’s Crosby England” nailheads, and 1995 and some 1996 (or at least certain saddles beginning with 5 and 6 as the first number in the serial number, that look to be '90s in styling and have the leather quality of their '90s saddles) have mostly black with gold lettering “Crosby England” nailheads. I’ve forgotten what the 2003-ish on have. The XL Excel, Selle Française, Corinthian, maybe the Ian D. Millar Vision? and a few others have their own nailheads.
I’m no expert, but love the older Crosbys and have dug around trying to research them, and have noticed a few things about them and made guesses. I wish there was a way to confirm if any of this is correct! I’ve tried reaching out to Walsall Riding Saddle Co., who made these saddles, but never got a response. Exselle, also made by WRSC, used to be able to make duplicates of older Crosby models and so I reached out to them as well with no response, and I think they’re sadly out of business now. There used to be someone on HGS that had a lot of informative posts about the older Crosby and Collegiate models, but those sadly disappeared with HGS.
As for the Centennial, from what I recall about those, they were Crosby’s answer to the Butet, introduced for the 1996 Olympics along with some other new models for Crosby and Collegiate (and those original 1996 models all have Olympic logos on them), came in a few different flap options, and the Centennial Internationale was their wool flocked option, while the regular Centennial was foam. The tree width should be stamped on the stirrup bar, but other than that, I’m not sure there’s a way to know about other specifications other than eyeballing and guessing. A lot of the Centennials I’ve seen seem to have the longer, straighter flap.
Sorry to be of limited help and to write a novel, but thought I’d share a bit of what I might’ve found!
Definitely agreed that it would be helpful to see the entire saddles! As for the model of the unknown one, does it have any writing on the underside of the sweatflaps? One of mine has the model both there and on the billet guards, though it may have also worn off by now.