It's possible to be a First Mate without that credential? I'll readily admit to being extremely ignorant about shipping, but I always imagined the position being akin to the US Vice President, in that part of the job description is to take over from the captain as necessary.
There is a specific Chief Mate license (according to STCW) but indeed commonly the Chief Mate holds a Master license, it’s just not required for minimum safe manning. (If it was, than the Manning regulations wouldn’t be met anymore when the captain is incapacitated)
At least here in Germany it only take ~3 years of sea service to progress from officer of the watch to master - but no one will entrust you with a major cargo or passenger vessel with that little experience.
