In Josef M. Schmidt's Pocket Atlas Homeopathy you can read:
"The formerly used name LM-Potency should be avoided because the Latin number LM doesn't mean 50.000 but 950.

He doesn't make any other difference between Q- and LM-potencies.
Siegfried

I've also checked in the biography of Samuel Hahnemann by Prof Robert Juette.
At pg 240 Prof Juette added to a phrase "... for what we call LM-potencies today":
"The Q-potencies are meant"

I'm still on the search how Remedia can claim about the difference between Q- and LM-potencies...
Siegfried

I really hope that I don't bore anyone, but I've found something else:

"To avoid confusion it needs to be pointed out that the terminology
around the 50 millesimal potencies has remained inconsistent.
It varies depending on the manufacturer, for historical
reasons, with some producers using the abbreviation ‘Q’ and
others the original name ‘LM-potencies’. The official German
pharmacopoeia Homöopathisches Arzneibuch (HAB) allows both
names and only makes sure that the manufacturing specifications
are consistent. The term ‘Q-potency’ (from Lat. quinquaginta
milia = 50,000) that is commonly used today traces back to
Jost Künzli von Fimmelsberg (1915–1992). Rudolf Flury
(1903–1977), who rediscovered Hahnemann’s 50 millesimal potencies
preferred the abbreviation ‘LM’ (from the Roman numerals
L for 50 and M for 1,000). ARCANA Dr Sewerin GmbH & Co
KG and other manufacturers therefore follow a tradition if they
use this name.

In the interest of consistency we use ‘Q-potencies’ or ‘50 millesimal
potencies’ in the following text (apart from in quotations)."

This quote is from "The LM potencies in homoeopathy:
From their beginnings to the present day" from IGM Institutsberichte