....That was a nice attention-to-detail move from FFC: During WWII, silk and synthetics were diverted from ties and other apparel for parachutes, etc....
The entire movie was a 'nice attention to detail'. In the case of this particular tie, I wouldn't be surprised to learn that this 'extra' was lucky enough to find & borrow it from his father's old trunk in the attic...which is why we see him placed in such a prominent place in the wedding scene.
Apple