Welcome aboard, Matthew!
the "5b" is probably "SB" as in sunburst, right?
Hopefully Hans Moust will chime in here and can look this one up in his records for you. If you don't already own it, pick up a copy of his "Guild Guitar Book" which is the definitive reference for this period of Guilds.
In the meantime, if you have any pictures of the guitar, especially the label and the back of the headstock, that'd help identify it completely. To post pictures on LTG you have to host your pictures elsewhere and link to them in your posts.
Again, welcome aboard!