This has been transcribed from a support email
I read the values of the meters back as configured on the Tools->Edit SNMP Server screen, with a Matrikon converter to convert from SNMP to OPC.
However, when I add a new meter, my logic does not seem to work any more. It seems like the sequence of the meter values become jumbled up, and then a meter reads into a different OPC register than before.

The problem is that your library indexes and addresses the meters in the sequence that they are returned, rather than using the meter serial number as an index in the SNMP address.
The sequence it is returned in, depends on the numeric sequence of the meter serial numbers.
To get around that incompatibility, we have made it possible to specify a different index for a meter, rather than the meter serial number.
The sequence the value is returned in, is in the sequence of the index, in other words.

Additional explanation on the above posts:

As written in another post about SNMP addressing, an SNMP address is normally something like: 1.3.6.1.4.32794.2.1.3.61500465
That addresses a specific data element in the meter with serial number 61500465, e.g. in this case (3) it is its Voltage on red in mV (e.g. INTEGER: 237347)
However, it sometimes happen when people pull in a lot of SNMP addresses from an SNMP device, e.g. PNPSCADA, that it sorts it in numerical sequence, and then allow you to specify a simple index to a long list of numbers to address the values, instead of using the proper SNMP address. As is the case with a Matrikon server we are aware of.
Now the problem comes in when you replace a meter. The place of the metering point in the indexed list used in the previous step could change, because the numerical sequence of the meter serial number could be at a different index all of a sudden.
To help customers suffering from this problem, you can specify an alternative address on the meter level for each meter addressed in SNMP on PNPSCADA, in the 'Index' column of the meter list in "Tools->Edit SNMP Server", as shown in the red circle in the following screen shot (some details has been obscured for the sake of customer privacy).


For example, if you specify an index of 777 for a meter with a serial number of 61500465, the SNMP address of its red voltage will change from 1.3.6.1.4.32794.2.1.3.61500465 to 1.3.6.1.4.32794.2.1.3.777

In that way, you can preserve your SNMP ordering. A correct way would therefore be, that you create the SNMP link for your meters normally, and write your logic in the Matrikon server etc., and then when you change out a meter, you change the index of the new meter to be the same as the index of the old meter.
Remember to restart your server for the new SNMP settings to take effect.