Cessna C210N, came out of the factory without main gear doors so had higher gear extension speed.
Cessna C210M, had main gears doors(unless had the door mod) and had a lower gear extension speed.
There was a website for a company in the USA(florida) that run a fleet of C210s on freight, which included a lot of information on variations of each type as well as a handling notes, ground school notes etc.