From Kurfurst's site:
The first AS engined aircraft were produced from December 1943, at the same period the production of the MkXIV Spitfires commenced.
Altogether 686 G-6/AS aircraft were built or converted from existing airfames, along with 76 G-5/AS, 16 G-5/R2/AS, 68 G-5/R6/AS, the only difference being G-5s having pressurized cocpits for the pilot`s comfort at high altitudes.
From spring of 1944 - and not during the summer as Williams suggests - MW 50 injection appeared on the Bf 109s, resulting two 'new' engines, the DB 605 AM with 1800 PS and a rated altitude of 4km / 13 125ft, and the DB 605 ASM with 1800 PS and a rated altitude of 6.5km / 21 000 ft. Both were essentially modified DB 605 A and AS engines (AS itself was based on the DB 605A-1, with it`s supercharger replaced by a larger one taken from the DB 603 G engine) equipped with MW-50 boost, boosting low altitude output up to the rated altitude, above which the MW injection ceased and performance was the same. These engines were built into both G-5/AS and G-6/AS aircraft, and was noted by Heinz Knoke`s war diary above. In addition, Olivier Lefebvre noted about conversion kits for the G-6/U2 subtype, which featered a nitrous-oxide booster (GM-1) tank, that could be converted to use MW booster with little difficulty :
"250 conversion kit were issued early spring 1944 for the already produced /U2 aircraft and switch on the production lines from GM-1 to /U2 occured at that time as well. The G-6/U2 convertion was standardized over the next few months, with replacement of the heavy tank with a thin aluminium one and some modifications in the MW-50 piping. While those aircraft were still produced as G-6/U2 the name switched to G-14 during summer 1944, the late G-6/U2 being identical to G-14."
From July 1944 the production of these improved G-6s was standardized, re-designated as G-14 (with medium altitude 1800 PS DB 605 AM) and G-14/AS (with high altitude DB 605 ASM), but otherwise differed in little from those late production G-6s. Some 1830 G-14/AS aircraft were built, bringing the number of 605AS and ASM engined G-5s, G-6s and G-14s to ca. 2500 examples. The number of G-14s built in total is unknown, an estimated about 3-4000 examples, the exact number being difficult to tell because of the many converted/repaired airframes.