While it's safe to assume that all systems with CPU frequency >8MHz used an asynchronous bus, I would think there was a time during the 8MHz era where OEMs had to weigh up the advantages of using synchronous or asynchrous, leading to a mixture of products that either were or weren't.

I have very little hands-on experience with AT-era hardware, but I'm curious to know if there's any way of working out when the first asynchronous board was introduced. I imagine a window of time is probably the best I can hope for. Any thoughts?