First of all, I will show the postmark in question:
It is on this cover:
I think it is not a machine cancel because the four bars on the stamp are not horizontally in line with the postmark. At least that is how I see it.
The letters "R.F.D." in the first row are new to me and I do not know what they mean.
I hope someone here can give some information about this postmark. Also to literature, in which I could find something about it.
Oh yes: still the back of the letter:
Have a nice weekend!