david weaver
Just a few responses to these

1 - why were the panel planes double ironed and why did that flow over to the smoothers? A lot of those were wedged, and it wouldn't be any harder for a maker to use a single tapered iron and a wedge, and if a lever cap is used, no harder to make a single parallel iron - actually easier, no slot is required and no cap iron.

2- Most older planes (including wooden planes in this), the vast majority still had laminated irons. All of the maker price lists I've seen show a single and double iron price if a plane is available with either, and in all cases, the double iron is more expensive. I don't think it was cheaper for anyone to make a double iron than a somewhat thicker single iron. By the time tool steel was common for all planes, steel was cheap.

3 - I have an infill panel plane that has the mouth filed at 9 thousandths (I put it together). I was so in the dark at the time that I didn't even file the top of the mouth toward the bun "away", so it essentially perpendicular - I consciously did that because I thought I would improve the plane by allowing the mouth to open less if the plane were lapped heavily by some future user (boy did I find out how unlikely that is when I lapped it to finish it). I did have to remove some material from the cap iron (which was fat and totally blunt), but it actually feeds very well and performs much better than it did before - it now has a 30 degree primary bevel and a fat 80 degree blunt about a hundredth or two, it is a common pitch plane. Not many older planes are that close, and I'm convinced that users would've known how to set them up. Most of the work I had to do to my cap iron was because the shepherd cap iron was crap, and my work to make it more gradual just makes it closer to the classic designs I've gotten as I've accumulated more parallel iron sets (I don't buy the whole plane, I'd rather make it). If I file the mouth away a little bit, then it will feed even better, but there is no need at this point.

6 - I've seen this. I'd love to have a talk with someone who did it, but they're probably all dead. It is pure guess, but I'd bet they'd tell us that they didn't need a mouth that tight for most work, and they could move the cap iron in close if they decided that they needed surface improvement.