Conventional wisdom has it that Hesse Darmstadt, also known as Hesse or properly as Hessen und bei Rhein, lost no territory as a result of the 1866 war
Hessen-Darmstadt did loose some territory, but to a rather moderate extent. Particularly the region Hessen-Homburg and the so-called Hessian Hinterland (an allmost isolated part) fell to Prussia.
The people from the former Hinterland also referred to themselves as "Muss-Preußen" (forced Prussians). To make things even more complicated, Prussia added the territories of eight more towns to the Hinterland that never belonged to it before, and their population rejected the term Hinterland for themselves.
Most of the territories in the Hessen region that Prussia won in the 1866 war (including also Frankfurt/Main and a few Bavarian border regions) were combined oin the new province Hessen-Nassau. This province, togather with Waldeck and parts of Thüringen, formed the garnison and the recruiting area for the XI. Armeekorps
I would suppose that only infantry recruits from the Grand dutchy Hessen-Darmstadt would have beared the Hessen coat of arms on the helmet. I.e., only units from the two Hessian brigades (49 and 50) and of course any additional units formed during the war.
My source lists the following IRs from these brigades:
* IR 115 Leibgarde-Infanterie-Regiment (1. Großherzoglich Hessisches) Nr.115
* IR 116 Infanterie-Regiment Kaiser Wilhelm (2. Großherzoglich Hessisches) Nr.116
* IR 117 Infanterie-Leibregiment Großherzogin (3. Großherzoglich Hessisches) Nr.117
* IR 118 Infanterie-Regiment Prinz Carl (4. Großherzoglich Hessisches) Nr.118
* IR 168 5. Großherzoglich-Hessisches Infanterie-Regiment Nr.168
And additionally following Landsturm/Ersatz IRs were established:
* LIR 115 Landsturm-Infanterie-Regiment Nr 115 (23.2.1915)
* LIR 116 Landwehr-Infanterie-Regiment Nr. 116 (2.8.1914)
* RIR 116 Reserve-Infanterie-Regiment Nr. 116 (2.8.1914)
* LIR 118 Landwehr-Infanterie-Regiment Nr 118 (2.8.1914)
* RIR 118 Reserve-Infanterie Regiment Nr. 118 (2.8.1914)
* RIR 221 Reserve-Infanterie Regiment Nr. 221 (25.8.1914)
* RIR 222 Reserve-Infanterie Regiment Nr. 222 (27.8.1914)