Chuck said
The No. 4 has an application code of S for Winchester.  

But any 3/8″ width will work.  Just like all the different app codes Lyman gave tang sights with the same Win hole spacing–any of them can be used on any Win with the same spacing; a way, I think, for Lyman to sell more sights.

Bert H. said
While “any” 3/8″ front sight will fit, or a tang sight as the same hole spacing, they most certainly will not all “work”.  The reason Lyman put an application codes on sights intended for a Winchester is that it determined what the height of the sight was/is. 

I don’t believe it. I don’t believe it makes any significant practical (as opposed to theoretical) difference.  True, if you install a Lyman tang sight coded for a M. ’90 on a SS it won’t have sufficient elevation to shoot at 500 yds, but that’s not what I consider “practical,” unless 500 yds is where you expect to be shooting. 

“Works” I define as conducive to shooting at reasonable iron sight ranges, 200 yds max.  I began shooting with any sights that would “fit” the slots & hole spacings long before I ever heard of an app code (like 50 yrs ago), or owned a Lyman catalog (though I’ve now got 20+ pre-war originals), & can’t remember any of my “mix & match” sight combinations that didn’t “work” for the shooting I was doing.

Bert H. said

This topic is concerning the application code on a front sight… and the height of a front sight is way more critical than the height adjustment on tang mounted peep sight.  I only mentioned the tang sight because you did.  The application code on a front sight identifies its height… try installing a front sight that is not tall enough… or one that is too tall, and then tell me “it works”.  

Of course it’s possible to run into a front sight of such extreme dimensions either way that a problem would arise, but most are in a “middle range” of height that will “work” with some corresponding adjustment of the rear sight.  My point was, don’t get hung up on app codes, as it’s not such a critical dimension that your eyes & common sense aren’t capable of judging, without knowing the app code, whether the sight will be usable; at any rate, it’s never been a problem for me.  The app code may constitute some theoretical “ideal,” but that doesn’t mean others can’t be used with satisfactory results.