I think the fly plaid (the dirk also btw) can be worn with a Montrose Doublet or a SherrifMuir Doublet..."white tie" events...Think royalty...Ascot...extremely rare high society stuff only.

Not sure about the Regulation Doublet...it's a white tie outfit and you can wear (miniature) medals on it.

Normal "dress" (Argyll Jacket) and "black tie" (Prince Charlie Jacket) events...no fly plaid, no medals and no dirk. We often see fly plaids worn with PC Jackets, but I guess that's because sellers want to sell us stuff.

It's a bit like the off white hose and the Ghillie Brogues I guess.

Okey...I know...I must run for my life now