Serial number milestones

Never in in the 37 years of Ercouping have I heard of the H. Why would they go back to a 75 HP? Where did you find this?

Wikipedia is a pretty good source for various versions, what major changes were made and number produced. It does not have AD info.It does not  have all the dates but does have number produced. I would guess as long as it stayed ERCO the ser # were sequential but that is just a guess.  BTW the -C stands for Continental engine.
The first production version of the 415-C NC15692 Ser#1 is at the Smithsonian. It was Manufactured in October of 1939.  There were 121 production prewar 415-C produced until January of 1941.  Production was stopped due to unavailability of Aluminum-Alloy sheet cause by the increase in priority of military production preparing for war and supporting Britain with lend-lease aircraft.
In October of 1945 production resumed.

Info from Wikipedia below:  I hope this helps.

415-C Ercoupe First variant to enter large-scale production. Prewar 415-C Ercoupes were powered by 65 hp (48 kW) Continental A65-8 engines, while postwar Ercoupes were powered by 75 hp (56 kW) Continental C75-12 engines. Maximum gross weight was 1,260 lb (570 kg). 4,520 total built; 112 pre-war and 4,408 post-war.

415-D Ercoupe Same As 415-C but with fuel capacity increased by 1 US gal (3.8 L) and a modified up-elevator limit. Gross weight was also increased to 1,400 lb (640 kg). 77 built. One 415-D was modified under a Supplemental Type Certificate to fit a 108 hp (81 kW) Lycoming O-235-C2C powerplant.

ERCO 415-CD Ercoupe 415-CD Ercoupe was same as 415-D but with a revised elevator limit as well as a modified nose gear and ventilation system. Gross weight was reduced back to 1,260 lb (570 kg). 275 built.

415-E Ercoupe was the same as 415-D but powered by an 85 hp (63 kW) Continental C85-12 or C85-12F engine, introduced the split elevator with 20° of up-elevator travel. Gross weight was once again increased to 1,400 lb (640 kg). 139 built.

415-F Ercoupe was the same as the 415-D but powered by a 90 hp (67 kW) Continental C90 engine.

 415-G Club-Air was the same as a 415-E but with a rear Kiddy-Seat and a modified windscreen.

415-H Club-Air was the same as the 415-G but powered by a 75 hp (56 kW) Continental C75 engines. A total of 70 415-G and 415-H aircraft were built.

Robert Burdett 415-C N99369

Before 813 they were welded steel on the front of the Spar. then Cast Alum from 813 up mounted on rear of Spar. while I am here Forney F1 was the first large rear Windows's Es had flat windshields so I think Forney's were the first bubble also. Some Forney's come with bucket seats later on. Also Forney had fold down seat backs  no idea when this  all Happened.C90's started with Forney.