Seat post Diamensions for Reynolds tubing ?

All seatpost sizes dependent on outer diameter and tube thickness

o.d gauge
753 28.6 0.7/0.5sb
731 os race 28.6 0.8/0.5sb
708 28.6 0.9/0.6sb
653 28.6 0.7/0.5sb
531c 28.6 0.8/0.5 sb
531st 28.6 1.0/0.7sb
525 28.6 0.9/0.6sb
525os 28.6 0.9/0.6sb
501 28.6 0.9/0.6sb
500 28.6 1.0
531 all terrain 28.6 1.0/0.7sb
531 magnum 31.75 1.0/0.7sb
501 all terrain 28.6 1.0/0.7sb
501 magnum 31.75 1.0/0.7sb
500all terrain 28.6 1.0/0.7sb
500 magnum 31.75 1.0

On hand built frame it depends who reamed (bored out) the seat tube, I have only done this once on a 631 tubed frame I built, it came in around 26.8 and the inside of the tube is not perfectly smooth so needs to be reamed out smooth to what ever size you want (within reason) I went for 27.2 using a hand reaming tool, bloody hard work.
cheers

One other point to consider is heat distortion. High heat during brazing can ovalise the seat tube.
Tony Oliver in his book explains that a meticulously-built frame will take a 27.4 post where a carelessly built one may have either a 27.2 or worse. Reaming thins the walls and is clearly undesirable.

Im pretty sure all frames are reamed to a certain extent, the inside of the tubes is not perfectly smooth after manufacture, or at least that's how it was explained to me on the frame building course I took, we certainly didn't try any seat posts before reaming and the tubes did not ovalise, also I would imagine the tubes can be thinner in the area the seat post is in with out loosing strength

We are talking of ovalisation of 0.1mm, small but enough to step down a seatpost size. There is a big difference between a quick lick with a reamer to tidy up and correcting a distorted tube. Remember 0.1mm is only 0.25% distorted.

My 1964 Reynolds 531 frame falcon road bike takes a 26.6 seat post
I have a 1980s falcon 501 frame and it takes something slightly smaller but can’t find a seat post for it

26.2 for example is quite a tricky size to find, try a USE post plus shims. It works fine, run my tourer like this for 15 years.

Thanks for reply
Will have a search

Try a really good old fashioned shop. They should have the conical gauge to measure seatpost size for frames.