What type safety did S&W Revolver have prior to WWII ?
I have 1915 S&W DA .455 revolver that I am wondering about.

I think a transfer bar.

An early hammerblock safety. It was activated due the hand and inbedded in the sideplate. This was not working very well. When it is dirty it wont work at all. That is why in 1944 the current L shaped hammerblock safety is placed.

The Victory model had a serialnumber started with a V. When a sailor dropped a Victory model, the hammer hit a hard underground. The revolver goes off and killed the sailor or an other. Smith and Wesson developed then the L shaped hammerblock safety. The new revolver was marked with VS. Victory Safety. After the war was the V dropped and the serialnumber started with S.

Smith and Wesson revolvers do not contain a transfer bar.

Some of these early revolvers did not contain the current style hammer block that cams on the rebound slide,and prevents the hammer nose from moving forward and contacting the ammunition/primer, or the hammer from impacting the firing pin as in the most recent frame mounted firing pin design. In fact, according to Kuhnhousen, M1917 revolvers with serial numbers prior to 185,000 were not slotted for hammer blocks at all.

Some early pre-WW2 models, however, although the hammer block we are familiar with now is missing by design, the internals did include what is considered the "primary" safety in these and our current day production revolvers, which is the interface between the hammer seat on the top of the rebound slide, and the rebound seat on the bottom of the hammer. While this interface does not prevent the hammer from reaching the primer when the hammer stud is sheared off, it does prevent the hammer from moving fully forward when there is an absence of full rearward pressure on the trigger, as when the hammer would disengage from the trigger during a dropping incident after the hammer is cocked, etc.

This interface prevents the hammer from going fully forward because the rebound slide reaches a blocking position under the hammer before the hammer fully falls, thus preventing the hammer from going fully into battery.
I'm not sure when this rebound slide/trigger return spring mechanism was first included in the full sized revolvers. Perhaps as early as the 1899 design. Perhaps others more familiar with this can comment further.

This safety mechanism can be tested by first safety checking and double checking...making the revolver safe (empty) and then after closing the cylinder and cocking the gun single action, inserting a pencil eraser end down into the barrel of the empty revolver. After inserting the pencil, the muzzle of the gun is held at an upward angle, and the trigger is struck (not pulled) with another rod, like a dowel or pencil. The hammer of course, will come down, and the trigger will come forward, but the rebound slide/trigger will fully return forward first, before the hammer, and the hammer seat on the rebound slide will prevent the hammer from going fully into battery..... and the pencil placed in the barrel will remain undisturbed.

This design in the mechanism works independently, and functions whether the hammer block flag is in the gun or not.

__________________
Ret. LE, FA Instr, S&W Armorer