I have a postcard (see attachments)
On the front side there is a postage stamp canceled on 15/05/1916 by the post office on the Allее des Capucines in Marseille.
The date corresponds to the event described in the letter - meeting of the Russian Expeditionary Troups on 21/04/1916.
The stamp is not clear, but the macrophoto shows that the stamp was canceled together with the postcard.
There is a little piece of paper on the stamp.
Obviously, the stamp was torn off the envelope and glued to the postcard before cancelation.
I think no one would have done so for collecting purposes and the card was actually sent and sent without envelope, but ... where is address ?
On the back side there is a blank piece of stamp - with its help, a lilac flower, referred to in the letter, was glued.
This is all I know and understand in this postcard.
How, where, to whom was it sent, if it does not have an address?
I think the stamp, possibly from the envelope in which the card was sent or unused from another envelope must have been cancelled by favour. Postcard collectors on the continent often liked the stamp on the picture side of a card I've seen stamps removed from the address side and stuck on the picture side but its obvious, there is a cancel on the stamp but none on the picture. Yours of course shows a cancel on the stamp AND the picture, which is why I say it was cancelled by favour, does it tie up with where the sender was? In WWI a postcard was often written standing up in the trench, a letter needed something to write upon, which is why so many cards are like this.
The envelopes were usually thrown away which is why they do not appear on sale with the postcards. It is simply an envelope with a hole to show the stamp and the address the postcard is going to to. When it arrived most recipients simply ditched the envelope.