Not many firm details on what German units were involved. Mention is made of Catholic Saxon and Bavarian Units. There is evidence other Germans States did not participate and so no truce along their lines.
One Leutnant Johannes Niemann of the 133rd Royal Saxon Regiment mentions being opposite the Seaforth Highlanders [2nd Battalion serving in the 10th Brigade, 4th Division] on the line Frelinghien-Houplines outside of Armentieres. Mention is made of the 2nd Battalion Dublin Fusiliers [serving in 10th Brigade, 4th Division] also enjoying the truce. Mention is also made of the 134th Saxons and the 107th Saxons. Source is based on information in Stanley Weintraub, Silent Night: The Story of the World War I Christmas Truce (New York: The Free Press, 2001).
Accounts from the 1/6 Battalion Gordon Highlanders [a Territorial unit serving in 20th Brigade 7th Division] held the line Sailly-Armentieres, only mention Germans, but they may be from the 134th and 107th Saxon Infantry.
Captain Stockwell of the Royal Welch Fusiliers [perhaps 1st Battalion serving in the 22nd Brigade 7th Division] mentions having Saxons opposite and meeting with them during the truce. They were apparently holding the line near Ploegsteert Wood.
The incident of New Years's Eve involved Henry Williamson of the London Rifle Brigade. The London Rifle Brigade was the 1/5th Battalion, London Regiment (a Territorial Battalion) which was serving in the 11th Brigade, 4th Division at the time. He mentions talking to Saxons and amongst them soldiers wearing a green lanyard with tassels whom he was told were Prussian snipers. His position was under the Messines Ridge. One source mentions that the Saxons were from Leipzig.
They must be from the XIX Saxon Corps, which appears to have held the line from Armentieres to Messines and perhaps beyond.
Hope you have a very Happy New Year.