Hi Alessandro,
What you have there is a "Leitwerksträger" - the lower tail part.
All the components of a Bf109 can be identified trough such plates, but you have to have a spare parts list (Ersatzteilliste).
The Sach-Nr. is used to identify the component - in your case 8 109 103
8 - means aircraft structural component ("9" are engines)
109 - the aircraft type, Bf109
103 - component number, Leitwerksträger
The Werk-Nr. below the Sach-Nr. is the individual serial number of the component and has nothing to do with the aircraft's Werknummer.
On the Bf109 G there are usually two plates with the aircraft's Werknummer. The main square plate located on the left side of the fuselage, slightly in front of the cockpit. And a second triangular one, positioned on the Leitwerksträger.
here's a link for an early G Ersatzteilliste
http://rapidshare.com/files/24123540/Bf-109G_Parts_Manual.pdf
regards,
Tomislav