Most of these engines don't have the horse power or model in the serial number but have distinctive Model G cast on the hopper.  The serial number is usually stamped on the top face of the head. From what I've found these were built from 1914 up to 1916 when the Super power line was introduced and changes made to the horse power.

3 1/2" Bore x 5" Stroke

300-600 RPM

Flywheel diameter 17" 1914

235 lbs shipping weight 1914

Most of these have the serial number stamped on the top lip of the head. There Serial number will start with 2.5G##### or 2.75####. These engines first appear in the 1913 sales catalog with governor weights in the cam gear "G1" and seem to have been short lived due to failure and switched to the speed screw style governor system know as the "G2" in the manuals and catalogs I've seen. From what I've found the 2.5 hp was built in 1913 then switched to 2.75 hp in 1914 along with the governor system. Also seems the water hopper went from a ornate top to a flat top water hopper at the same time.  From what I've found these were built from 1914 up to 1916 when the Super power line was introduced and changes made to the horse power.

4 1/4" Bore x 5 1/2" Stroke

300-600 RPM

Flywheel diameter 22" 1913

450 lbs shipping weight 1913

Most of these have serial numbers stamped on the top lip of the head or on top lip of the block where the head meet. There serial number start with 4G#####. These engines started off in 1913 with the G1 governor and changed to the G2 Speed screw in late 13-14 but did not change horsepower like the 2.5 did.  From what I've found these were built from 1914 up to 1916 when the Super power line was introduced and changes made to the horse power.

4 3/4" Bore x 6" Stroke

300-600 RPM 

Flywheel diameter 26" 1913

750 lbs shipping weight