Hard to find and becoming ever harder.
Bailey moved his plane making business from Winchester to Boston in 1861 and began producing this line of bench planes with an adjustable frog.
I even noticed on Home Depot that Stanley is still producing the 5 jack plane.
These are getting difficult to find.
However, the patent drawing for the change shows what some believe is the real reason for the change - the circular disk, on the lower end of the lateral adjustment lever, loses its ability to engage the slot provided for it in the cutter when the iron is nearly used up.
It took Stanley nearly 50 years to realize this.
Hope this helps with your plane type I.
Displayed where it will not be examined too closely.