As Iccapland has noted above, Byrum was on the march from Stalag Luft IV. There are other letters written by Stalag Luft IV prisoners that have that "odd" return address.
Here is a link to some letters from Raymond E Bence, a Stalag Luft IV POW, which show the same camp address as is on Byrum's postcards. http://notalice.com/capstone/assignments/letter_pow_19450110_raymond_bence.pdf
Bence's name shows up in the (incomplete) Rose roster of Stalag Luft IV. http://www.stalagluft4.org/sorted%20roster%20cup.html
so there is no doubt he was there. Stalag Luft IV shows up as the holding camp on his POW record at archives.gov, same as Byrum.
I don't know why the "return address" is not Stalag Luft IV, but I don't think in these cases, that "Kriegsgefangenenlager der Luftwaffe Nr. 3" meant they were in "Stalag Luft III". I also don't know if the letter "B" which is written on all the letters near the address means anything. It might just be a censor's mark.
post edited by RSwank -