Within an oval on the steerer tube, there are four lines stamped as follows:
A & P
I'm not certain about the "A" after G13, and the ellipses (...) indicate that there are additional characters that are too faint to read.
Interpreted From Classic Lightweights UK site information:
"A&P" means the steering tube was Accles & Pollock tubing. "BGT" should be "BST" for British standard thread. "G13A..." could be "G/13/16" which equals "...Gauge (wall thickness is 13-standard wire gauge one end and 16 the other = 2.3mm thick at the crown and 1.6mm thick at the top, threaded end (as all modern steerers)." B.. should be "BQ" means "B" quality tubing.