In Genesis 9 a rainbow is a symbol of a covenant. Cooper also refers to this.
It is also tempting to think of the Royal Ark Mariners, but Day already thinks to have found that in the next panel.
Cooper presents an interesting “Seal of the Grand Chapter at York. c.1780” with a rainbow resting on clouds, a triangle below the rainbow (one point up) and a crescent moon with the points up below. I have not yet found that image elsewhere, nor have I found information about this Grand Chapter of York. It would be interesting to find out if this was an “Antient” Grand Chapter and if perhaps William Graeme’s lodge in England or the Stirling lodge in Scotland had affiliations with them.
A rainbow is a not too common Masonic symbol, but it can be found on a third degree “Masters Carpet” from the 1920 as can be found on the Phoenix Masonry website (scroll all the way down).