Hi John, good to see you here again :-)
Yes, you do appear to have bought a mystery watch. ( I was watching the auction )
I've seen a few watches with these markings on the side, but can't honestly say that I know what they are. As you say, they are not the markings of the 20th year anniversary model. ( unless there is an undocumented version that we don't know about. )
Maybe however I can give you some Info on the others items / fact that you mention.
The Box
I have 2 versions of this box as shown here.
One is from 1988 for a limited edition Seamaster as shown by the label on the box cover - this definatley dates the box to the late 80's
& the other has "normal" Speedmaster Pro markings on it ( as on the case back )
I've seen this box "claimed" to be issued with 2 different speedmasters. One was the "Grail" ( alos an 80's watch ) & the other was the 20th Anniversary model. The next picture is used in both Omega SAGA & the "new" book "A Journey Through Time"
So the box is looking for good for a 20TH anniversary model.
Now onto .
.
The Owners manual
I have 3 of these. All marked differently inside the rear cover. One of mine is CLEARLY marked from 1989. ( it has the text - "The watch the world has learned to trust" - in WHITE on the back cover ) - The others are not so clearly marked.
the 1450 Bracelet
According to "The Master of Omega", the 1450 was one of the Bracelets available in the 1980 - 1995 period. The 1450 is certainly one of the bracelets that was available in 1989.
.
Everything is gearing up for this to be, as you say, a late 80's watch. ( or even a 20th anniversary model ? )
Did the seller give you any extra information ? Can you ask him for more Info ?
I'd love to know what the movement number is ( the 1st 5 digits will do ) & to know if the Museum will give you any further Info. A quick tip, if they don't / won't give you any Info - order a "Certificate of Authenticity" for the watch. That will give you the manufacturing date & country the watch was sold to :-)
Please come back with more info when you get some.
All the Best
Steve