No, the "A" was an older straight-guage tubing by Reynolds, and it seems to have been completely eclipsed by 531 EXCEPT for some Spanish bikes (like Zeus) and the occasional Australian. It's often compared with Accles & Pollack tubing from the same early era. I'd guess that being straight guage it would use a smaller seat pin, so maybe 26.4 is right...having the tube measured is your best bet.