Ok as I see it,
a per records like I would see in my home database:
to me it is the Marriage index number of the database used.
in my databse:
[RIN ###] the individual index of the database
[MRIN ####] marriage index of the database
in this databas my number is RIN 1 my husband's RIN 2 and our marriage is MRIN 1 as they are the first records I put in.
They are usually local to the database and not relevant anywhere else.
The page may have been created by a database and the inclusion of those numbers could have been a default.
Anyway that is how I see it.
NG Hill