Canadian No. 5 Mb&lu With US Stamp And British FPO

This cover is from a Canadian serving with the Mobile Bath and Laundry unit No. 5 in Italy. It is dated 3rd August 1944. I have always liked these items from second line - non combatant units - that were vital in supporting the fighting troops.

I am familiar with Canadian units using the British style double ring F.P.O. cancelers. This one is FPO 524. But I have not found an explanation of the American stamp cancelled by the British canceler other than the writer had a US stamp that he put on the envelope and then handed it in to the Canadian postal system who accepted the item.

Any other thoughts would be welcome.