What does this symbol (placed on the shield being held by an angel figure) mean? Is there any historical explanation for what it represents?
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The square and spear are emblems of St. Thomas, the Apostle, aka Doubting Thomas.
He was well known as a builder in his lifetime, though I have my doubts that he participated in all the constructions listed on the site linked to above. This explains the builder's square in the emblem.
St. Thomas was stabbed to death by the spear represented in the emblem, c 72 AD.
This shield emblem might represent St. Thomas himself, or have denoted that the bearer regarded St. Thomas as a patron.
Here is a link to a page of similar symbols for emblems of Thomas the Apostle.