Is there a citation or (at least a date) to ascribe to Conway's Circle Theorem?

Question

There are lots of John Conway tributes (via cardcolm.org) appearing online, and one of the things that I see a lot is the fact that his Circle Theorem (via MathWorld) is a geometric construction that in principle could have been known to the Greeks but was not discovered until Conway found it. I have yet to see an actual date or a citation to his original work.

This is the theorem:

Given $\triangle ABC$, extend sides $a$ and $c$ through $B$ by length equal to side $b$. Similarly, extend sides $b$ and $c$ through $A$ and sides $a$ and $c$ through $C$. Then

1. The ends of the extended sides lie on a circle (the Conway Circle); and
2. The center of that circle is the incenter of $\triangle ABC$.

Is this just a party trick or conversational observation of his that was only published by others later on, or is there an origin date? The earliest date I've seen (in the linked cardcolm.org page) cites someone who provided a proof in 2013.