Sir William Wallace Information?

I have an interest in william wallace. according to legend, when wallace gave up the role of guardian of scotland he called a meeting in the ruins of selkirk abbey on the outskirts of Errick forest. here he proclaimed that two guardian's should take his place, one being Robert the Bruce and the other (the Bruce's enemies) John Comyn. According to the book 'The steps to the empty throne' and 'The Wallace' by Nigel Tranter, Wallace took the great seal of Scotland and broke it in two pieces, handing them out to Bruce and comyn. My question is....How true is this? does that same seal survive today? and if so, where can it be seen?