I don't think it's possible to make real sense of either. They probably never existed and are simply misinterpretations of drawings of armor.I direct everyone to another egregious bit of the PHB.
... So what's the difference? Leather is AC 11 + Dex modifier. You sew rings onto it, and it becomes either Studded Leather (Light Armor, AC 12 + Dex modifier, 45 gp, 13 lbs.) or Ring Mail (Heavy Armor, AC 14, 30 gp, 40 lb.)? I told my players it's the quantity of metal sewn on that determined whether it was Light or Heavy, but then I tried to imagine how unwieldy it would be to sew 30 pounds of metal rings onto a suit of leather armor and wear the result.
However, looking at 1e, I think we can make at least a LITTLE more sense of it:
1e DMG, p. 27:
1e DMG, p. 165:Studded leather is leather armor to which have been fastened metal studding as additional protection, usually including an outer coat of fairly close-set studs
Plates set in the leather to me suggests something like a brigandine, which had metal plates riveted to cloth or leather, but it's hard to square that reading with the first description.STUDDED LEATHER adds protective plates set in the leather and an extra layer of protection at the shoulder area.
Along with this latter description, Gygax recommends as a source Charles Ffoulkes' Armor and Weapons (1909), conveniently in the public domain and available on Project Gutenberg. Therein, we can find this image
Number 3 is a model of supposed 'trellice' armor, about which the author says
Sounds a little like studded leather. And Number 5 is supposed to be ring mail.Another method of reinforcing the leather defence has been named the ‘Trellice’ coat. It is always difficult to discover exactly what the primitive draughtsman intended to represent in the way of fabrics, and it is quite open to question whether these diagonal lines may not merely suggest a quilting of linen or cloth. If it is intended to represent leather the trellice lines would probably be formed of thongs applied onto the groundwork with metal studs riveted in the intervening spaces(Plate I). This arrangement of lines is very common on the Bayeux Tapestry.