Also from November 1943 onwards, all-metal return rollers of a few different types were used due to lack of rubber supply. Zimmerit anti-magnetic coating to protect vehicles from magnetic mines was applied starting in September (MIAG facility) or November (Alkett facility) 1943 and ending in September 1944. In 1942, a variant of the StuG Ausf. F was designed with a true howitzer instead of the 7.5 cm StuK 40 L/43 cannon. These new vehicles, designated StuH 42 ("Sturmhaubitze 42", Sd.Kfz 142/2), were designed to provide infantry support with the increased number of StuG III Ausf. F/8 and Ausf. Gs being used in the anti-tank role. The StuH 42 mounted a variant of the 10.5 cm leFH 18 howitzer, modified to be electrically fired and fitted with a muzzle brake. Production models were built on StuG III Ausf. G chassis. The muzzle brake was often omitted due to the scarcity of resources later in the war. Alkett produced 1,299 StuH 42 from March 1943 to 1945, the initial 12 vehicles were built on repaired StuG III Ausf. F and F/8 from the autumn of 1942 to January 1943. In 1943, 10 StuG IIIs were converted to the StuG III (Flamm) configuration by replacing the main gun with a Schwade flamethrower. These chassis were all refurbished at the depot level and were a variety of pre-Ausf. F models. There are no reports to indicate that any of these were used in combat and all were returned to Ausf. G standard at depot level by 1944. In late 1941, the StuG chassis was selected to carry the 15 cm sIG 33 heavy infantry gun. These vehicles were known as Sturm-Infanteriegeschütz 33B. Twenty-four were rebuilt on older StuG III chassis of which twelve vehicles saw combat in the Battle of Stalingrad, where they were destroyed or captured. The remaining 12 vehicles were assigned to 23rd Panzer Division. Due to the dwindling supply of rubber, rubber-saving road wheels were tested during 8–14 November 1942, but did not see production. Bombing raids on the Alkett factory resulted in significant drops in StuG III production in November 1943. To make up for this loss of production, Krupp displayed a substitution StuG on a Panzer IV chassis to Hitler on 16–17 December 1943.