John J. Robinson claimed in his book (born in blood: the lost secrets of Freemasonry, 1989 ISBN 0871316021) that there’s a similarity between The Masonic Square and Compasses and Solomon’s Seal and the reason for this is that the Masonic Square and Compasses are one of many Masonic “loans” from the Knight Templars.The order of the Knight Templars was founded in 1119, and it’s connection to Solomon (and Solomon’s Seal) is apparent from its full name: The Poor Fellow-Soldiers of Christ and of the Temple of Solomon. Their first headquarters was at the Temple Mount (The Temple of Solomon
The Masonic Square and Compasses appear (mainly in North America) with the letter G representing Geometry or God or both. With or without any connection to the Square and Compasses symbol the letter G appears in the center of Masonic Solomon Seals (six-pointed stars).
Picture is copied from Wikipedia entry Freemasonry (uploaded by MesserWoland).