You may all know about this already, but in case you didn't.
I've just bought new inlet and exhaust valves (53 plunger A10) and they appear to be the same size and aren't marked in any way☹️
Just found out how to tell them apart. The exhaust valves are Austenitic steel and non magnetic. The inlet valves are magnetic. Saved me a lot of trouble.
Presumably, these are modern exhaust valves designed for unleaded fuel - they need to use ~SS material to stop valve seat recession.
I'm hoping so, but I've been told that it applies to older valves as well, due to the behaviour of Austenitic steel at high temp (not something I know about though).
It has certainly been the case for many years - 50 that I know of and probably many more.