Mullard xf3 codes

Hi!

I have come across Mullard EL34 xf3 coded tubes. They have a shield logo on top, a BVA logo, the legend "MADE IN GREAT BRITAIN" and codes xf3 and B 304 below which are, however, not etched into glass but printed in chalk ink. The sound is very good (smooth, uniform and detailed), but are they real Mullards?

Best regards,

Alex