This specimen belongs to the D series of the Murer and Duroni strut folding cameras, which corresponds to focal plane strut folding cameras without cross and rise movements. The shutter speed range (1/31 minimum speed) shows that it was made before the new shutters delivered from around 1925. The format 6 x 4,5 (model DU) was the last to be added to the series around 1916. 
The body is made of wood and aluminum covered in leather. The viewfinder slides vertically over the lens on the front standard. The focal plane shutter is a Murer, delivering speeds in the range 1/31 to 1/1000. The non-self-capped shutter has constant tension, and the speeds are set by slit width, which is controlled from outside but checked on a scale on the internal back side (real speeds values inscribed and not slit width).
The lens is a Rex Luxior (Paris) with a focal length of 70 mm and maximum aperture of 5.5. This lens may indicate that the camera was commercialised in France - the equivalent Murer lens would be the Anastigmat 5,5, corresponding to model DU-E.

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At Murer and Duroni, Klapp cameras are divided into two major families: those with a focal plane shutter and those with a central shutter. They are then classified using a nomenclature consisting of 2 to 4 letters or numbers.

Focal plane shutter:

First letter		Second letter				Third and fourth letter
D	Fixed front face	U			4,5 x 6	2	Anastigmat	Murer	f/7
F	Platine avant à décentrement	N			6,5 x 9 ( O from 1914)	M	Anastigmat	Murer	f/6,5
		L			9 x 12 ( E from 1920)	T	Anastigmat	Murer	f/3,6
		C			9 x 14	E	Anastigmat	Murer	f/5,5
		K			10 x 15	R	Anastigmat	Murer	f/4,5
		H			13 x 18	0	Aplanat	Murer	f/8
		W			45 x 107 mm (Stereo)	Si	Anastigmat	Koristka Sideran	f/4,5
		S			6 x 13 (Stereo)	Ke	Anastigmat	Koristka Equator	f/5,5
						Ka	Anastigmat	Koristka Meridian	f/6,8
						K	Anastigmat	Koristka Doppio Gauss	f/5,5
						3	Anastigmat	Goerz Dagor	f/6,8
						4	Anastigmat	Sintor	f/6,8
						B	Anastigmat	Berthiot Olor	f/5,7
						S	Anastigmat	Suter	f/5
						L	Anastigmat	Roussel	f/4,5
						Z	Anastigmat	Zeiss Iena	f/4,5
						1	Anastigmat	Zeiss Koristka Tessar	f/6,3
						5	Anastigmat	Primus	f/7

Central shutter:

First letter		Second letter		Third letter or number
U	4,5 x 6	L	Plate (simple)	0	Aplanat	Murer	f/8
N	6,5 x 9	F	Film-pack	2	Anastigmat	Murer	f/6,8
G	9 x 12	G	Film	3	Double anastigmat	Goerz Dagor	f/6,8
W	4,5 x 10,7  (Stereo)	P	Plate and film-pack (premium)	M	Anastigmat	Murer	f/6,5
S	6 x 13 (Stereo)	T	Plate (economical)	S	Anastigmat	Suter	f/6,8
I	6 x 13 panoramique	M	Plate (metal)
A	4 x 5
O	4 x 6,5

Klapp cameras with a central shutter can come in three different finishes: economical, standard, or premium.

In the "economical" version, the body is made of alder wood and covered with imitation leather. The viewfinder is integrated into the front plate. The shutter offers only P (Time) and I (Instant) settings. The shutter is of the sector type. The metallic parts are oxidized. Its lens is achromatic. There is only one model of this type, which is a UT.
In the "standard" version, the body is made of aluminum and brass covered with morocco leather.
In the "premium" version, the body is made of aluminum and covered with morocco leather. A mechanism allows for moving the front plate forward to focus from 1.5 meters to infinity. The viewfinder is mounted on the sliding lens cover. The shutter speed can be adjusted to 1/25, 1/50, and 1/100.

(source Malavolti)