There will be nymph flies you have that do not exist anymore and you wish to have tied but either you do not tie or have no one to do it for you. Just by taking a good photo of the fly with a description of the materials should be enough to match for you.  It can be tied with a photo coming back with the nymph for your approval before commencing an order. I do this now for a few customers where they have purchased some flies but either they have gone out of production or cannot remember where they have purchased from. Fly patterns do change all the time and in retail it is hard to stock everything due to cost and in most cases flies that do not sell often get specialed out and replaced by something else. You guess it, it will be one of ones you purchased that works for you for the river system you fish in. Do not hesitate to send an email and a request and I will let you know if it can be done.