I bought this at a flea market today for 10. It is 11.5 inches with a 1.75 blade. According to the blood and gore site, that makes it a 5 1/4 jack produced from 21-83. However, using a plane dating flowchart I get different results. There are zero patent dates, no raised ring, no plane number on bed, a recessed lever cap, and a frog receiver with a broad rectuangular area with arched rear -> which dates it a type 4. However, according to rexmill, the type 4 doesn't have a lateral adjustment lever, which this does. These also seem to have a different frog style. It has nothing cast on the bed except Stanley at the front. The lateral adjustment lever has nothing on it and it is two piece with the circle at the attachment. The adjustment knob is brass with 3 rings.
It has a sweetheart blade with made in usa smaller than the notched box surrounding the Stanley.
I realize that the cap, blade and breaker may not be original. I have a number 4 type 15 that has the same style frog with the frog adjustment screw.
I'm guessing from the frog and blade that this is an early sweetheart model less the adjustment screw, but the bed castings are inconsistent with the information available maybe because this is cited as a model designed for schools.
Any help would be appreciated. Thanks.