So I accidentally noticed that the cast iron number 5 base from the 1930s is larger than the cast iron base from the late1940s, early 1950's.
The early bases are about 1/2 an inch deeper and 1/4 inch wider.
I try to be up on all the Jensen history but this completely took me by surprise.
I've attached a photo.
PS I am in need of the larger #5 cast iron steam engine if anyone has one lying around. Let me know.
It is like the light blue one in the top of the photo.
Cheers