I bought a bridle belt from a company called Dubarry of Ireland that came with the buckle. It's a brass quick release. Dubarry no longer carries it but Equus Leather now offers a belt with a near identical buckle. Remembering where that buckle came from send me down memory lane; the top Google image result for "ludlow quick release" is a picture I posted to SUFU in 2008!
I don't mean to pile on you Diggsy as the overall outfit is a fine weekend look. The jacket is a bit boxy as suggested, and this may be due to the proportions being off. Look how low the pockets and lower button are. It looks like a longer jacket has been over-aggressively shortened., either by you or the previous owner of you thrifted it.
Good eye! I was trying to hide the shortness of the right sleeve length. The jacket has fake surgeon button holes and two of them are up inside of the sleeve! I'll work with it.
I see two takes on casual business dress- NiceGuyTom's, which isn't doing it for me (black oxfords and french cuffs don't lend themselves to casual easily), and NewYorkRangers, which does: the black shoes are boots, the shirt is a OCBD, and the cardigan stands in place of a coat.