The full monty
The Field marshal? Unlikely. The phrase doesn't appear in Ngram until 1976 and didn't pick up in popularity until the mid 90s. John le Carré used it in The Tailor of Panama. It was published 1996 but the story refers back 20 years and used in the context of Burton's the tailors. Montague Burton was the founder and a if you ordered a three piece suit it was "the full Monty". The usually reliable etymologist Michael Quinion also says that this is the most likely source.
Not definitive then but seems reasonable.