On the 1872 era Newhouse, Andreski and Parr mention a keyhole eye on the #0, #1 and 1 1/2 traps. What does this refer?

I am not so sure that 1872 is an exact date for the beginning of the key hole spring eye. In 1865 Oneida community added the #0 and #1 1/2 sizes to the Newhouse trap line. About the same time and likely before, they started making some of their trap springs from sheet steel, rather than hand forging them, this was in response to Hart making trap springs from sheet steel in 1864. The lower eye on the forged spring was round and the jaw post was bolted to the base and the first traps with machine cut springs were the same. But in order to save time and materials, the jaw posts on the #0 and #1 traps were riveted to the base rather than bolted. The new springs had the lower eye cut in a key hole shape so that when the jaws were removed, the spring could be turned at a 90 degree angle and it would slide over the jaw post. This was nessasary so that a week or broken spring could be replaced. The #1 1/2 remained the same as earlier with the round hole and bolted jaw post. Most all #0 and #1 newhouse traps with machine springs have the key hole eye from 1865 ish, until they started using the Victor style base and springs in the mid 1920's.

That is what I thought it was, since the 0, and 1's, and 91's, and 81, I have all have that feature. In the book it was mentioned once and not again. Since there was no reference picture I wasn't quite sure about that.

Thank you