I just find it interesting that Aquila generally holds two spears, and these eagles are always clutching something.

Perhaps it is a sun reference. I could see it with the outstretched wings and all.

Aquila is a symbol you don't see used very often, especially by Christians. The only place I've seen it there specifically is the floor murals in the church of St. Catherine of Siena (which are quite different than the norm).

One occult drawing has Aquila figuring prominently over an alchemical cave that Adam and Eve (or Sol and Luna) are entering. That's a really crappy version of that engraving...but it is an interesting one.