Jérôme
(Bill and Jim suggested I reposted my follow-up to David as a new thead, so here it is, with edits.)

> ...with larry and mention of the double iron, I saw only two references anywhere else to practical use of the cap iron before 2012.

As you (David) often say yourself, "I am not trying to sell anyone anything", but I find old books interesting and google books allows us to access some ancient knowledge.

SALIVET, 1796

For instance, the "manuel du tourneur" (wood turner manual), 1796, by Louis Salivet, a Roubo contemporary. In this book, he attributes the double iron to the Germans and recommends to set it so that the iron protudes "imperceptibly", at most "quarter of a line" (1/12 of a French inch, say less than .5mm). He mentions that the double iron must bent square then honed to meet the blade (That would represent a final ~80° angle ?).
The expected benefit is explicitly to reduce tear out.

He also goes on to mention that one can get the well prepared adjustable screwed iron+double iron sets at “Bergeron". This could mean that the practice was well known at the time.

See:
- whole book : https://books.google.fr/books?id=Ssk9AAAAcAAJ&dq=Double+fer+rabot&hl=fr&source=gbs_navlinks_s
- page "iij" (after 467) : https://books.google.fr/books/content?id=Ssk9AAAAcAAJ&hl=fr&pg=PR3&img=1&zoom=3&sig=ACfU3U3bnLHxmgfMQ4ZtvSaL8BqkLKn5cA&ci=74%2C386%2C745%2C807&edge=0 and https://books.google.fr/books/content?id=Ssk9AAAAcAAJ&hl=fr&pg=PR4&img=1&zoom=3&sig=ACfU3U0wIhPC5metd3YnZH4PiqWPjqAW3w&ci=236%2C117%2C702%2C104&edge=0

NOSBAN, 1857

In a more recent book ("Nouveau manuel complet du menuisier de l'ébéniste, du layetier, du marqueteur, du sculpteur", 1857, by Nosban, ") the author says that for difficult woods, with a wooden plane, the wedge could be inserted between the iron and the cap iron, as to increase the angle. But he also goes on to find this technique too inconvenient. He also mentions that the tip of the cap iron is "rounded" (unicorned maybe ?)

See :
- Whole book: https://books.google.fr/books?id=Zu0OB-uKo8MC&hl=fr&pg=PP13#v=onepage&q&f=false
- page 181 extract: https://books.google.fr/books/content?id=Zu0OB-uKo8MC&hl=fr&pg=PA181&img=1&zoom=3&sig=ACfU3U0rNk1wXXu7jnYAxdUra-JMsflxhQ&ci=42%2C94%2C819%2C653&edge=0

ROUBO,