10K White Gold Filled - M5 marking on case back and movement - 2590 stamped on case inner plus watch maker servicing marks. Silvered dial with diamond markers with exposed balance.
Wow, this one is interesting! Can you confirm date code on movement? It looks to me like it might be "L1"?
Pre 1967, this watch with a matte finish dial with no cutout was a Beau Brummell and post 1967 was renamed Diamond Excellency.
The skeleton polished silver dial is something I have never seen. My initial gut reaction is that this is a "one off" jeweler modification of the dial.
In reply to Wow, this one is interesting… by neetstuf-4-u
There is an M5 stamped next to the "B" in Bulova, there also appears to be an M1 stamped near the end of the 'Barrel Bar' after the 'Ratchet Wheel'.
Agreed, 1965 Beau Brummell, noting "non-conforming"(for lack of a better term) dial customization
1965 Beau Brummell
Likely the Beau Brummell 'AP' originally in 1965
LB128 1965

1965 Beau Brummell "AP" match