If the Salisbury copy (one of only four surviving) was produced in Salisbury, it stands to reason that it is not a Base or Generation Zero document but is a Generation One copy of the original. It seems probable that one of the original copies produced at the time of the Magna Carta’s signing in 1215 was being circulated and each authority wanting a copy had to produce its own copy by having a scribe transcribe it. Then, as a means of quality control, it was sent to the King for affixing of the Great Seal was a way for both sides to certify that the copy was a true copy of the original document.
This is a efficient and economical means of laying off the significant cost of producing the handwritten copy on those who wanted one but at the same time ensuring it would be accepted by both the owner and the Crown as a true copy for present and future reference. I image there was probably a hefty fee associated with obtaining the Great Seal on the document.
By the way, except for the Great Seal quality control bit, hand copying of important documents and books was the standard way (prior to the printing press) in which scholars in both the West and the East built their personal libraries.
Among the Chinese, it was also the practice to interlace the document’s text with their own commentary on it. Maddening to disentangle at times to obtain the original text without distracting asides, etc.