Even though it can be considered a partial grill, the hard stops up and down are obvious, and can be seen equally well on the front. They measure out to a perfect 10mm, in which case it would be an "I" grill.
Lengthwise gets trickier. It's hard to tell from photos but with the naked eye the stop can be seen at 8.5mm.
If it did reach the length of an H grill at 12mm, it would go way off the stamp. (Would that be irregular? I'm not sure)
Just hoping to get people's opinions on partial grills, as well as this piece in particular.
Also, is the 183 shown in the first picture a Chicago precancel?
Thanks in advance