snmp unknown object identifier

I'm trying to access a TP link access point to get some basic data from it.
According to TP Link, I need the rfc1213 mib, which I've downloaded and put in my ~/.snmp/mibs directory
However :
$ snmpget -v 1 -Cf -c public $IPADDRESS rfc1213.mib::ifInOctets
rfc1213.mib::ifInOctets: Unknown Object Identifier
greping on the file shows that I do have entries for ifInOctets.
Changing ifInOctets to something else (e.g. Time ticks) gives me the same result, but using the long number (1.3.6.1.2.1.1.3.0) works.  As I'm not sure what ifInOctets translates to, I can't readily use the number instead.
Having spent half the afternoon rummaging around on the 'net with no clear (to me) answer, I'm here.
Suggestions on where to continue welcomed.
Thanks
Adrian
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