We have a some AdfWeb M-Bus snmp devices. This devices provide read snmp data as hex data block with .0 or specified byte with .X.  This device allways return value in one step up snmp index in tree. Look this pic

But async snmp agent poller cannot read this

SNMP OID: get[enterprises.33118.1.1.1.4.1.0]

Old classic snmp poller works with it without any problems

You can read official snmp manuals on page 29-31

http://www.adfweb.com/download/filefold/MN67163_ENG.pdf