I've been shooting the Mod. 16-4 I bought a few months ago.

Have cleaned it several times too..but just noticed the revolver has a trigger stop slot in the frame..however there is no sign of the stop in the slot!

Did S&W cut some frames for the trigger stop..but not install the stop?

Thanx Stevie

Very likely...or else someone took it out. I've always removed and discarded them.

My guess is they did make them with the slot and w/o the stop installed. My near new 14 has an empty slot as well.

+1

All of my K and L frame revolvers have the slot with no stop.

Yes.
Near the end of 1978, S&W stopped shipping target K-frames (KT) with the trigger stop. KTs that had the TT/TH continued to ship with stops into the early 1990's.
It wasn't until about 1994 that the slot was eliminated from the frame, the same time that the new style rear sight assembly came out.
Before that, all KT frames were cut for the stop.