@algor
First of all, welcome, Alberto! I recognize your username from G503.com, even though it was several years ago. You and I have had many pleasant discussions there, as well as email, and even some "horse trading"! I am Wingnutt on G503.com.
I didn't see this thread and discussion when you originally posted it.
I would like to know the difference between a Stanley 100 screwdriver and a 100 Plus.
It would help to know the context for your question and your terms. Are you trying to outfit a certain kit? Are you seeing references to "Stanley 100" and "Stanley 100 Plus" in an SNL or TM or other period document?
Based on catalogs, "Stanley 100" is not a term they used as a branding device in the vintage era.
They used the term "100 Plus"
for decades, since the 1920's at least, well before WWII, and they continued to use it throughout WWII, and after WWII - for their
top of the line highest quality
wood-handled screwdrivers.
The part number was No. 1001 for the standard blade, in various sizes. No. 1003 for what they called a "small" blade, referring to what are known in other nomenclatures, including military, as "electrical" or "cabinet" style blade. Note that they made other wood-handled screwdrivers, including "Hurwood" and Regular, which were lower quality and price.
Some time between 1935 and 1937, Stanley introduced the "Stanloid" line (with composite handles). Part numbers were similarly No. 100X scheme, depending on type (Standard, Close-Quarter, electrical, etc). The "Stanloid" line supplanted the "100 Plus" line in quality and price point, but it did not replace them. Stanley continued to produce the "100 Plus"
wood handled screwdrivers throughout WWII and well after, even after they introduced other composite-handled lines (e.g., "Victor" and "Hercules").
In 1953, Stanley started branding their top line composite-handled screwdrivers (formerly branded "Standloid") as "100 Plus". Note that they were
still making the wood-handled "100 Plus" line at the same time!
HERE is a link to the Stanely page of the Internet Archive International Tool Catalog Library. It contains literally hundreds of Stanley catalogs, brochures, price lists and other supplemental material from 1855 well into the modern era (1990's), including the annual 1941, 1942, 1947, 1949, and 1953 catalogs, all with Screwdrivers sections, which are the catalogs you will want to dip into. I have the page sorted by date. Just scroll down from there.