Stirlingengineforum.com

Kudos, Vincent, you nailed it except for noting that the offset crank (aka desaxe) means that upstroke and downstroke are "irregular" from the get-go. Check out this video on the most famous "side lever" engine and jump to 2:30 for fun part:

https://www.youtube.com/watch?v=VWn8gQ9Ykpk

Another dwell scheme is 2 bellcranks connected by compression spring, but I've never been able to find a cad video of such.

Ahh don't think I would have caught that detail. So yea I'd love to hear his design process.