I notice that Ian has not said the date of the marriage in question.
If it was before 1855, the parish register will not contain the signatures of the couple. In any case, you could download an image of the original entry at home so there is no need to go to an SP centre to view it.
If it was after 1854, as Imber has said, you will not get anything extra by going to the NRS if you already have an Extract.
If the couple were married after 1854, they may or may not have signed their names in a parish register. Never having seen such a register, I do not know. But if that is what you are hoping to see, the NRS is not where you will find it. You would have to track down the register of the church whose minister performed the ceremony. The GROS had no power to call in Church of Scotland parish registers after 1 January 1855, or to call in the registers of other denominations (Free and dissenting churches, Episcopal Church, Roman Catholic Church) at any time.
If the marriage was less than 75 years ago, you are not allowed to photocopy, photograph or save to a memory stick the image of the marriage certificate, so even if the certificate had the couple's original signatures, your helper would have no means of showing them to you.
My understanding of the process of registration is that the couple obtained from the Registrar a Schedule of Marriage. They, the witnesses and the minister completed and signed the Schedule and returned it within three days of the marriage to the Registrar. The Registrar then copied the details from the Schedule into a pair of duplicate Registers. At the end of the year he sent one Register to the GROS in Edinburgh, and retained the duplicate. The Register sent to Edinburgh is the source of the images on SP.
If you need what is generally referred to as a marriage certificate, you can apply either to the NRS or to the local Registrar, and what you will get will be a document with the formal title 'Extract Entry of Marriage'. They are usually typed these days, though before the advent of typewriters they were of course handwritten. (I don't think I have actually ever seen an 'Extract Entry of Marriage' as my parents' wedding was not in Scotland and I don't have any older ones.)
The Schedules of Marriage were also sent to Edinburgh, but I do not know whether or where they were stored, and if so for how long. As far as I know they are not available for research, but I would be happy to be corrected on that point if someone knows better than I do.
