As you have found, Retrospect names its tapes with a sequence number and the Backup set name.
The pre-labelled tapes you have will be the barcode labels intended for use in a library or auto loader. Since you have a standalone drive with no barcode reader, the tape labels are irrelevant.
I would simply get some sticky labels and relabel the tapes according to the Retrospect numbering scheme.
You can add them all to the backup set in advance by inserting the tapes one by one, and clicking the ADD button, and then write the label for the name that Retrospect gives for each tape. It's a bit laborious but you only have to do it once.
This scheme works perfectly for me