The secret's out! I've been making parts for Marklin and other engines for years, and you will find, more often than not, that it's an oddball American thread that will best match what was used on pre-1930s engines from Germany. Metric is more common later. Best to have a large selection of taps and dies for both, not to mention good thread gauges.
I agree on lots of them, my collection also includes many from the UK, the ME taps I mention, BA set, and BSP set.
I need to make that cap. Originally it was likely nickel plated. That would be way too new looking to match the patina. I am thinking of using SS 304 or nickel rod as a start then try and age it to match with chemicals and heat. Any tricks in your bag for this, maybe a different material or ??
Bob