Read my article on German Anti Tank Tactics in the Landships main site. By 1918 heavy machine guns were begining to be regarded as an important anti tank weapon and provided with K rounds, After Amiens they were supposed to regard anti tank as their prime duty.
The German tanks (A7V and Beute) carried about 3.000 rounds per machine gun, thereof roughly 25% = 750 SmK (steel core bullets). Note that the SmK rounds that Mitchell ascribes to the German tank(s) did not origin from 561 "Nixe" or any other A7V. These were infantry rounds, most probably from the forward elements of Garde-Grenadier-Regiment 5 (i.e. Garde-Füsilier-Bataillon).
Guderian puts the 228th Jagers, 4th Guards and 77th Jagers Reserve at this battle with the 77th Jagers Reserve being in the area of the British Mk IVs. IS Guderian's ACHTUNG-PANZER a reliable source?
I also read somewhere that the 6pdr had a faster ROF fire than the 57mm. Is this true and by how much?
Guderian talks about the divisions: 228th Infantry Division, 4th Guards Division, and 77th Reserve (Infantry) Division, the three divisions that attacked together with A7Vs on 24th of April 1918.. The Mk.IVs that met 561 "Nixe" were in the area where 4th Guards and 77th Reserve had their divisional border (Guards in the north facing the Bois d'Aquenne, 77th in the south facing Cachy). The 57 mm had a rate of fire of 20 - 25 rounds per minute. No idea for the 6 pounder.