Has anyone got an explanation for this ?

[Big John]

Allegedly, according to Google, if the internal diameter of the seat tube is 27.2 it could be Reynolds 531. No guarantee but it could be. Many years ago I bought a bike frame & forks from a guy in Derby which had a Reynolds 531 badge on it. It was brand new and wrapped in bubble wrap. Some time later I had to take the bike into the LBS for some work only to be told by the shop owner that it wasn't 531 because the internal diameter of the tubing wasn't 27.2mm. I'd been turned over. He said the frame was ok though and probably worth the money I'd paid for it. I ride it to this day.....rides like a dream and no clue what tubing it is.

 

[keithmac]

If it's just for aesthetics you could buy some brass rod of appropriate diameter, slide it in the holes, peen it (hammer it) flat and then dress with a file. Basically like an old school rivet but with the ends dressed off.

No heat required.

 

[Drago]

If you weigh as much as I do you'll want them filled proper, like.

 

[sasquath]

As a matter of interest ,how do l tell what the steel is ? l have stripped the paint off so can post photos of bare steel if that would help.

On dropouts you could try spark test search on YouTube.
Another weldability test is to poodle a spot with Tig torch and try if it hardens, if it does it's a high carbon steel and pree/post heating is required.
Tubes, no idea - spark test is too aggressive, and possibly too crude to differentiate between cr-mo steels.

Most reliable way would be to identify the frame and dig out old manufacturing data, or to send shavings to a lab