Internal measurement is 26.8, ie seat post is that size. Have just checked to confirm. The 26.8 seat post won't fit in 531 Pro but the markings on that post not readable but seem to think was 26.4.
Lots of these frames were chromed to end of seat stays and part of right hand chain stay, and the forks. Some painted over by Peugeot: some people have stripped the paint off. My 653 Athena (1990) has chrome under the fork paint (the purple colour way).
They used Simplex drop outs - the serial number sounds definitely Peugeot. They had chain pegs to bottom of seat stay, gas escape holes at the bottom of each of the 4 stays and 2 each on inside fork blades, top and bottom. Tried to attach link to the Peugeot catalogues but seems to be down. Thought I had the 1987 catalogue, turns out to be 1988 UK. That year they used 753 and 531 Pro and 531 with an ATB and 531 ST with some touring frames. If you would like it, send me a PM and I'll attach it and send (it's a PDF).
Good luck.