Apparently you can buy postcard backing paper that you stick on the back of the 4x6 print. This gives you the same backing as on Ilford postcard paper and is probably easier to write on. It can of course be added to any paper of your choosing. It adds stiffness to it so is probably the equivalent in stiffness and weight to Ilford postcard which is Portfolio paper.
This would be a perfect solution, Pentax! I am thinking of using Slavich Unibrom 160 but it tends to curl so much, flattening would be a problem. A stiffer backing would solve that and I don't want to have to get out the glue stick.
Sourdough, salami and blue cheese... and 2 dogs drooling with such sad, sad eyes. ... they're working me... they know I'll cave!