Image from Youtube
Go to about 1:30 in the video to see the saw guide.
I am helping my son build a pinewood derby car, and something like this would be helpful.
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I do not know the specific make of that guide but it is known as a hand mitre saw.
For example, from here:
(The one in that image appears to resemble a vintage Stanley mitre box, maybe a Stanley 358.)
There are other styles of mitre guides and boxes, too, the most familiar probably being the little plastic box with angled slots in the sides.