I am almost certain that what you are lucky enough to have is an older Stanley Sweetheart number 5 1/4 jack plane and you can confirm that by checking the width of the sole, which should be about 2 1/8 wide. I don’t think the early ones had numbers cast into the base like the later ones did and the hole in the lever cap started as a keyhole shape in many Stanley planes but were changed to a kidney shape later on. While it is possible that the blade was swapped from another plane of the same width, it isn’t probable and the Sweetheart marking on the blade is the only difference between a Sweetheart and non-Sweetheart plane (in the old planes) anyway. The 5 1/4 was produced and used primarily for school woodworking classes because the narrower blade and lighter weight made it easier for students to use. The market it was being produced for promoted a bare-bones but sturdy type of design but they can be excellent planes. Notice the single piece, twisted, adjustment lever (instead of the brazed two piece) and painted tote instead of rosewood. I would guess that it dates from around 1925 give or take about five years. The corrugated sole version is more rare and is therefore worth more but many collectors would like to have this one.