Kornel,
Reddemann in his book: "Geschichte der deutschen Flammenwerfertruppe" (from which I quoted in my earlier post) is contradicting Heinrici.
During most of 1915, there where no 12 companies of Flammenwerfer. Only when the III. Btl expanded to regimental size a 12th coy (and others) was (were) added.
A bataillon of Pioniere had four companies. The I. Bataillon would have coys 1 - 4. The II. Bataillon 5 - 8 and the III. Bataillon 9 - 12.
That was also correct for the III.Garde-Pionier-Bataillon before it became the Flammenwerfertruppe.
Once Reddemann established the III. Garde-Pionier Bataillon as the Flammenwerfertruppe, in the spring of 1915 the the company numbers changed from 9, 10, 11, 12 to 1, 2, 3, 4 (since there were no other Flammenwerfer companies). Therefore, in 1915 the former 12.Kompanie of the III. Garde-Pionier-Bataillon is indentical with the 4.Kompanie III. Garde-Pionier-Bataillon (Flammenwerfertruppe)
Heinrici quoted the company numbers of the III. Garde-Pionier-Bataillon like any other III. Bataillon.