Can "Mullard" tubes be labeled as RCA, GE, etc?

They will always say "made in Great Brittain" if they are mullards.

No tubes are "best". It depends on the circuit you are using them in and on your hearing preferences. On the other hand, Mullard and Telefunken, also the Amperex bugleboys are the most popular (and costly )amongst audiophiles. Mullard has manufactured for a lot of other tube makers and Elevick is right, generally it says made in Great Britain, which by the way can be ( and often is) easily forged. Mullards also carry a code near the bottom of the tube, for example Xf1 for EL34s or 7L1 for E88CCs.

The particular tube I'm asking about is a 5ar4/gz34 rectifier.
Your comments are appreciated.

for the 5ar4, if they are real Mullards, look inside the glass and the inner plates should look like a razor blade and yes, they can be labeled GE and RCA, and they will always be made in Gt Britain.