Where are you seeing references to different tang sights for the 73? I have never seen such references.
I know very little about the 1873 Winchester.
I know even less about tang sights.
I thought it would be no problem finding a tang sight for my gun.
The problem apparently lies with there being about 6 different clones of the 1873.
And, it's not clear if or what clones followed the specifications of the original 1873 to the letter.
When I went to say - Brownells to find the sight, the sight they listed under Winchester 1873 was specifically for one of the clones.
Looking at several sites, I came across a warning that the sight I selected was for a 22 degree tang and wouldn't work if the tang angle of the gun was different.
Based on this warning, I revised all the sites and found the tang angle for the sights varied from 18 to 22 degrees.
Some sights for the Winchester 1873 did not list the angle.
Further perplexing me was that apparently the tang angle for an original 1873 is 18 degrees.
But I measure the tang angle of my 1873 to be 22 degrees.
The spacing between the screws is not important because Marble used a slot to allow for differences.
The problem is the tang angle:
Mine appears to be 22 degrees when Marble says their sight for the 1866/1873 is 18 degrees.
It is apparent that the sight post is supposed to be perpendicular the the rifle bore.
If it is 4 degrees off, you will be looking either above or below the front sight when looking through the peep sight hole.