The "I" is for Ideal (grade). They didn't start marking the string of numbers denoting optional features until later. The other possibility would be that it was sent back to the factory after initial delivery to have the Hunter One Trigger installed. It seems this practice was not uncommon, as they didn't charge any more to retrofit it than they charged to install it as an option when ordering a new one. I think it was $25, but that may have been early-on.