Usually, when I produce a video tape for a client, there's a job number printed somewhere on the tape. It'll be a series of about three to five numbers and letters, probably on the bottom of the cassette, on the label, or the cover (e.g. 346D2). The same job identification will be printed on invoices, too. This identifies exactly what job the tape belongs to, and allows me to find the right master tapes to produce another copy, very easily. Alternatively, I can look up names and dates in my records, but the job id. is preferable.
Once you've determined a way to positively identify what you need duplicating, please contact me with the details and your requirements.
Please note that I generally only keep master copies of my own video productions. I won't have master tapes for making duplicates of other work, such as film and slide transfers, or copies of client's video tapes, unless the job called for it.