"Pea shooter" design based on the Raoux-Millereau popular in Great Britain in the first half twentieth century.
The following manufacturing details are provided by kind courtesy of Dr. Arnold Myers and listed in the Journal of surviving Boos?, Boosey and Boosey & Hawkes brass instruments, with archival and catalogue data :

112981; pistons 87741 (Military model french horn, 3 valves; inscribed on bell with monogram "B & Co" / "``Sotone'' / (REGISTERED)"; on garland "Boosey & Co Ltd", trumpet-and-banner trademark incorporating "TRADE MARK" and "BOOSEY" / "112981", "Makers London"; on 2nd valve casing "32" / trumpet-and-banner trademark incorporating "TRADE MARK" and "BOOSEY" / "87741").
According to the firm's archives [pistons books], the pistons were made by E. Holmes.
"F & E-flat Fr. Horn A41" $3.
Given out 4 Jul 1921; received 31 Aug 1921; made up 9 Aug 1921.
Paid for batch of 24, Holmes 202 hrs @ 1/11¹/₂d = ?19/15/7 + Stevenson 10 hrs @ 1/4¹/₂d = 13/9d + Beer 18 hrs @ 1/8¹/₂d = ?1/10/9; ?22/0/1 charged to Regent Street.
According to the firm's archives [instruments books], the instrument was made by Bloomfield.
Polisher, H. Owen; grinder, W. Cave.
"F & E-flat French Horn", Model A41.
Given out 9 Aug 1921; received 20 Oct 1921; given to polisher 9 Sep 1921; grinder 22 Nov 1921; charged to Regent Street 18 Mar 1922.
Paid for batch of 6 (112978-83), Bloomfield 119 hrs @ 1/11¹/₂d = ?11/13/0 + Morley 107 hrs @ 6d = ?2/13/6 charged to Regent Street.
[ Private collection, U.S.A. ]