It is of course not the improved one from Harald Wolf...

"1927. The year
1928 already saw the Schüler Company in busines with its
model "Jumbo”. Amongst the international importers were
companies like Charles Heyer & Co., the leading gunshop of
Nairobi / Kenya, Groenefeld & Hicks of Pretoria / South Africa,
Etablissement Janssen, the sporting arms supplier of the
Action details ofone of my .500 Jeffery rifles.
Engravings by Monika Wolf.
Belgian King in Brussels, a company named Hollander in New
York and Maison Didier of Saigon, in the then French Indochina
(Vietnam).
At the same time the world-renowned British firm of W.
J. Jeffery introduced an absolutely identical cartridge, called
the .500 Jeffery rimless. Who came first, Jeffery or Schüler, is
difficult if not impossible to say. Personally I tend to believe that
the .500 rimless is a brainchild of Richard Schüler. Most of the
Jeffery guns and rifles were actually not made by Jeffery himself,
but ordered at other companies. So maybe there was a
cooperation between Schüler and Jeffery planed at an early
stage. However, the .500 Jeffery rifles were definitely not supplied
by Schüler." Text from Harald Wolf:
http://www.hataritimes.com/d/waffen/e_jeffery.html