1917 Harley-Davidson Messenger Pigeon Carrier was used in the movie "The Spirit of St. Louis" after its use in WWI. Subsequently, it was bought by Steve McQueen without the Pigeon Carrier attachment.

Dale's Wheels Through Time found the machine in 1984 and then in 2003, found the pigeon carrier attachment. 

When put them together, everything fit perfect, and there were actually parts from the exact motorcycle hidden inside the carrier wagon.

https://www.facebook.com/wheelsthroughtime/photos/a.10152090524542901/10158610410587901/