Hi Stu
Not too much to add but can report that the 1st Army Corps began the war as part of the Field Army under the Command of General Baron von Manteuffel.
The Infantry component of the 1st Army Corps was as follows, 1st Infantry Division, 1st Brigade, 1st Grenadier Regiment (Crown Prince), IR 41. 2nd Brigade, 3rd Grenadiers and IR 43. 2nd Infantry Division, 3rd Brigade 4th Grenadiers and IR 44, 4th Brigade 5th Grenadiers and IR 45.
It fought in the battle of Columbey-Nouilly on the 14th of August 1870 where it fielded 24,830 troops and suffered the loss of over 2800 casualties.
The Battle of Noisseville August 31-1 September 1 1870. 2292 casualties
During the Siege of Metz to the capitulation of Marshall Bazaine and the French Army of the Rhine, the 1st Army Corps suffered the loss of an additional 187 men.
15 November 1870, under the command of General Baron von Manteuffel, 1st Army to the end of the War.
The Battle of Amiens 27th of November and subsequent actions up to December 15 1870, 961 casualties
The Battle on the Hallue 23-24 December 1870 and subsequent actions up to December 31 1870, 214 casualties
January 1-31 1871 Engagement at Robert-le Diable-Maison Brulet and subsequent actions Tertry-Poeuilly, St. Quentin, 1144 casualties
The return of 1 March 1871 lists the combat strength of the 1st Army Corps as 22,336 men, 1194 Cavalry and 94 guns, with a total strength of 40,252 which includes noncombatants.
An interesting table lists the maximum percentage of sick during the war of the 1st Army Corps at 20 per cent, and on the 19th of February 1871 12 percent.
My information source was the German General Staff Report on the Franco-German War 1870-71 which happily was translated into English by a countryman of yours around 1874, Major F.C.H. Clarke. This five volume set was reprinted by Battery Press and can generally be found available on EBAY. A great source..
Perhaps your helmet was picked up on the field, hence the year scratched into the spike…
Hope this is of a help, I looked through the tables fairly quickly (about an hour) so may have missed something but hopefully this should give you a general idea of the activities of this Corps.
Cheers..
Larry