That is a trigger stop for the single action function that you describe, and it is removable. It is a holdover from My Daddy's day when Bullseye shooters actually used revolvers.
There were four triggers that were offered with the K and L frame revolvers. A narrow serrated, a.400 wide smooth, and .400 serrated ,and the even wider serrated target, which is what is pictured in the link. The .400 smooth was offered after a lot of ppc and distinguished shooters bought the .400 serrated and ground and polished them smooth.
There were also three hammers available as well, the standard narrow hammer, the wide hammer (pictured in add) and the wide target, (even wider than the one pictured)
From what I see in the gun posted in the gunbroker link, the Hammer is a standard wide, was pretty much what came on most 2 1/2" and 4" 19's and other K's late 70's on. However it does not match, or is it the same style as the Wide Serrated trigger that is installed in the pictured gun.
Indicates to me that the trigger was swapped out, and more than likely the original hammer remained.
The hammer and triggers from both K and L frame revolvers were interchangeable.
Older K's and those made as basic police or military service guns, the M-10, fixed sight, the M-13 in .357, and the M-58 in 41 mag all USUALLY shipped with the narrow triggers and hammers. The Adjustable sight Models, the 15's, 19,and 57 shipped with the wider hammers and triggers. There were some target models, one the 25-5 comes to mind, as well as the M-57,and the 29's shipped with the really wide hammers and triggers.
A lot of the K',L, and most all N's with barrels 6" and over shipped with the really wide "Target" hammers and triggers.
We used to swap them out to fit the individual shooter all the time. Most of the time it was a pretty straight drop in that required very little fitting or stoning to get it dialed in right. . still have a few of them laying around somewhere..
It would nether ad or detract from the value of the gun.. I hope this doesn't confuse.