Randy, Maybe I can help with the l and M series. l - Early l's (1972-74) used a 130 HP 301 cid engine. These were underpowered when used with a hidro, so in 1975, AC introduced the 158 HP 426 cid engine for use with the hidro. The 301 continued to be used in belt drive machines. The bin capacity was rated at 120 bu on the early l, with no extensions, and I believe around 150 bu with the small box-type extension. The factory bin extension which extended out to the sides, beginning possibly in 1973, but for sure by 1974, yielded 180 bus. rating. You are correct on the 48" cylinder. Rigid headers could be had in 13'-24', and the Series 1 and Series 2 flex heads (made by attaching a Hart-Carter cutterbar to the head) ranged from 13'-22'. The Series 1 used a pitman sickle drive, (corresponds to a HC Series 2) and the Series 2 (corresponding to the HC Series 3) used an open wobble box drive. M - Early M combines (1973) used a 120 HP 301 cid engine. later, beginning in 1974, I believe, the horsepower was increased to 130 HP. The bin capacity was rated at 105 bus. with no extensions, and 165 bus. with the factory large bin extensions. Your 40" cylinder width is correct, and the same heads were used on the M as on the l. The only difference was that 22' was the maximum rigid head size. The only difference in the head was the location of the lower locking hooks. They were mounted to the outside for the l and to the inside for the narrower feeder house of the M. l2 - Early l2 combines (1977-78) were virtually identical to late (1976) l machines, except for simplifying the cleaning shoe drives, larger air cleaners, and improved monitoring of drives. These used the same perforated bin extensions as the later l's, and are primarily distinguishable by the use of a white stripe decal on the bin sides, as opposed to the black stripe decal on the late l combines. Of course, they have the l2 model designation. In 1979, (model year, introduced in late 1978),the bin configuration was modified, using a solid bin extension, floating bin loading auger, longer unloading augers (optional),full-range cylinder drive (no more changing belts and sheaves!),2-speed chopper drive, and probably more improvements. The bin capacity climbed to 200 bu. The 145 HP 301 engine option was also offered at this time. This configuration continued through the end of the Series 2, in 1982, and even carried on through the l3 series. The rigid headers were the same as the l series, until introduction of the 300 headers (1982IJ). The Series 2 flex headers were superseded by the Series 3 flex headers (AC built),then by the 300 headers. Sizes were the same. M2 - Most of the differences regarding early and late l2 combines also apply to the M2. Beginning in 1979, the 145 HP engine was also offered in the M2, primarily with hidro drives. The late M2 bin capacity was rated at 180 bu. Header sizes remained same as the M, although the series followed the l2 series headers. Hope this helps fill in some gaps. larry