The Mainz Kreis regiment was created from companies taken from Mainz infantry regiments Wildenstein and Riedt in 1756, by an agreement between the Archbishop of Mainz and Austria. It was organised on the Austrian pattern, and consisted of 2 battalions with 6 companies each and 2 companies of grenadiers, plus a garrison battalion. The regiment was disbanded at the end of the war in 1763. Although not a part of the Reichsarmee proper, it saw a lot of action serving with the Austrian Corps which fought alongside the Reichsarmee for much of the war.
These are my very speculative flags for the regiment; we know nothing of the appearance of the real flags, if any were carried.
The regiment spent much of the war serving alongside the Austrian field army so saw a great deal of action. At Prague on May 6th 1757 two battalions of the regiment, stationed just south of the gap in the Austrian lines where the Prussians broke through to win a hard fought victory, suffered heavy losses of 409 men. After the battle roughly one battalion joined the garrison of Prague and another battalion continued serving with the Austrian field army. It was at the battle of Moys on September 7th, when an isolated corps of the Prussian army was successfully attacked and its commander, the somewhat sinister Winterfeldt, who was a close friend of Frederick, was killed, possibly shot in the back by his own men. Later that year this battalion was at the siege of Schweidnitz and then at the battle of Breslau on November 22nd in the Reserve Corps. Having become part of the garrison of Breslau after the battle it was captured when the Prussians recaptured Breslau on 21st December but the men were later exchanged.
In 1758 the battalion with the Austrian field army was involve din the battle of Hochkirch on October 14th as part of the column under Lieutenant Colloredo that attacked the village from the east. On November 20th 1759 the regiment was at the battle of Maxen, where a detached Prussian corps was destroyed. In 1760 the regiment was with the Austrian corps serving with the Reichsarmee and was involved in the combat of Strehla on August 20th, where a smaller and more agile Prussian force thwarted a Hochkirch-style envelopment by the allies. In 1761 and 1762 the regiment was still in Saxony serving with the Austrian corps attached to the Reichsarmee. The two field battalions were at the embarrassing defeat of Doebeln on May 12th 1762 then both served with their grenadier companies at Freiberg on October 29th. Campitelli's Corps of which they formed part put up a good fight against the northern Prussian attack, although the battle was decided elsewhere on the battlefield.
And this may have been the uniform in use in 1756 (although there is much uncertainty about it):
