The early seventies is also the time the VW aircooled type 1 was at its latest form (the dual-port VW1600 engines, introduced in 1970) and the type 4 was just introduced as a even further improved engine for bigger, heavier cars (VW411/412, VW-Porsche 914 and the VW Bus (Transporter)) in 1700, 1800 and 2 liter form for more performance and reliability.
The aircooled VW boxer engine was more or less one of the most used and produced engines and applications in its day. No wonder there were also many companies that made tuning parts for the platform, both in Germany and the US. There was even a popular single seater racing class, that started mid-sixties with VW Beetle engines, transaxles and front axle: the Formula Vee class.
Early seventies, the Formula Vee (Vee from Volkswagen) series finally adopted the 1700cc type 4 engine by reducing its volume to under 1600 cc from a smaller bore so it could stay in class and use the slightly more reliable type 4 engines. These cars with even further upgraded suspension (A-arms instead of torsion bars) were called ‘Super Vee’. Many well-known racers started their careers in those FV cars, amongst them Arie Luyendijk, a later Indy 500 winner.

Its my own deduction of events, but when I measured that the (one and only) and very probably very first, casting Apfelbeck made of his own type 4 OHC cylinderhead, had a bore of just 90mm (1700 engine bore), that it was probably meant for the Super Vee class with a reduced stroke crankshaft! Apfelbeck mentioned his heads would have made a specific power per liter of 115hp, which would have put a 1600 type 4 Super Vee engine at about 184hp, which would have been condiderably more then the later introduced watercooled 1600 VW Golf derived engines, which ran a few years in side by side with the aircooled engines, but operated at about 170hp at the time!