I’ve never heard of a anti-reversion device employed in a Schnürle scavenged (ported) engine. That’s not to say it wouldn’t serve that function, but the reality is it likely doesn’t serve that purpose well at all considering the Schnürle port arrangement. The port arrangement and timing sequence pretty much causes the A/F charge to make a loop going up towards the glow plug, bouncing off the top of the chamber, and basically shooting towards the exhaust port. I suspect it’s one of those things where they use the same piston and liner in many engines and with certain stroke lengths and/or crankcases, the piston ends up being below the bottom of the exhaust port in the liner at BDC. I have many engines that have this “feature”, but none of them have “pipe timing”.
The old cross scavenged engines had a baffle machined into the top of the piston that, when the transfer ports opened, the fuel mixture hit that baffle and went up toward the glow plug(s). (Some engines employed dual glow plugs)