The 503B is a different beast than the old 503 boat anchor.
I'd suggest buying one. I had a 503 and got rid of it when the opportunity came to pick up a good condition 503B.
The drift is minimal after warm up and a few degauss cycles. The 503B tells you when it feels a degauss is needed and you just unclamp, close the clamp, press the button, listen to a bunch of clicks and clacks and you are good to go.
I've had the 503B on for many hours and have not seen drift issues. Of course with any magnetic probe/clamp, if you are after accuracy you need to realise that external magnetic fields (even the orientation versus planet earth) need to be addressed in your degauss/probe position.
Anyhow, I'm very happy with the 503B and have been using it for 5+ years without problems with an A6302 probe.
cheers,
george.