This is how I’m reading it.
The main Dia 4 hole is on the centre of the entire part. This hole goes through the entire component and I’m assuming this is where the wire fuse would pass through.
The centre of the semi circle is offset - based on the dimensions he has supplied.
As you said, there isn’t a good reason why it should be, but maybe in 1924 when the engine was manufactured they liked to be quirky.
Right, I need to get back to my actual job 
I can see that’s possible, but it still seems unlikely to me.
I won’t print anythig until you’ve checked with your friend.
He’s just replied!
Yep, his drawing is incorrect. They should be 19.5mm.
It’s a good job I’m not modelling and printing it
The overall size is not critical as he has to make copper contacts for the ends. Any adjustments will be made to suit.
I’ve noticed on your model that there is no hole going through the “spool” component. The D4 hole in the ends needs to travel all the way through.
Other than that, we’re good to go 
I did notice the missing through hole in the central component, and added it after I’d uploaded the draft model.
Ok now. I’ll have a go at printing one this afternoon.
How many copies does he need? Two? More?
Just two.
Are you sure you don’t want any payment? I’m sure he’d be willing to send beer!
Quite sure. Doing it out of interest. I’m retired, and don’t need money for the trivial cost of plastic filament.
I’ve had three failures when trying to print. Windows Update interrupted printing twice and shut down the print process so I couldn’t resume. So I ran the updates, and tried a third time. This one aborted for no obvious reason, not even finishing the raft for the print.
Trying for a fourth time, with a different arrangement of the model elements to avoid the need to print support.
So far so good, the raft has finished and the parts are now a few layers above it. Probably over an hour and a half to finish, if it doesn’t stop again.
Well, I’ve now had two successful prints. Here’s the second one still on the printer bed.
Three pieces arranged so they don’t need support.
I’ll cement them together this afternoon.
The first set printed on a raft, which is hard to remove fully. So I just used a (thinner) brim for the second print.
Great job by @john_mcclenahan! Thanks for posting the details.
They look great! Thanks for everything you’ve done. Once he has finished the project I’ll get him to send some pictures to post here 
