This gun is what collectors call a 1902 1st change. The important characteristics
of this variant is the 4-screw frame (which implies the early lockwork), the lug under the barrel to secure the front end of the extractor rod, and the shoulder on the barrel
where it meets the frame.

The model of 1902 (no-change) has a serial number range of about 21000 to 33000.
Its primary difference from the model of 1899 is the front locking lug under the barrel.
Around 33000, the threaded portion of the barrel, and the frame nose, were made
heavier. This resulted in the shoulder on the barrel, where it meets the frame.
As a result of this, the patent date roll markings would not fit on the top of the barrel,
so there were moved to the right side. This was only necessary for the 4" barrel.
They remained on the right side until about 1916, when a new roll-marking die was
made that would fit the space on the top of the barrel.

This heavier barrel thread and frame nose was the 1st engineering change to the
model of 1902, and so it is called a 1902 1st change.

At serial number 62450, the cylinder stop was redesigned. This engineering change
resulted in a 5th frame screw, placed in front of the trigger guard. This is the
introduction of the 5-screw frame, and this gun should (but is commonly not) known as
a 1902 2nd change. This recognition was delayed until about serial 76000 or so,
when the lockwork was redesigned by incorporating the new trigger rebound mechanism.

At serial number 58000, a new model - with a square butt- was introduced. This model
is known as the model of 1905. Except for the butt configuration, the gun is identical
to the round-butt model. For collectors, the model of 1905 causes lots of confusion,
because while being engineering-wise identical to the model of 1902, it is a new model.
Thus, it has no notion of engineering change associated with its name.

That is, a comparable round-butt model, like yours, carries the 1st-change designation.
An indentical square-butt model, perhaps only a few serial numbers away from yours,
does not carry such a designation. These two models were made in the same serial
number series all the way to the beginning of WW2.

Mike Priwer