So far, the only one I have seen with a stop like that is your post about the Woodpeckers square.
I have bought Woodpeckers products, but only when no other option was available. I find their tools very well designed, but overpriced. They are frequently the superior tools in their class, but often you can find something that does the main job for 1/5 the price. The less expensive traditional tool will do the job just as accurately, but without the small conveniences. I would rather live without those conveniences and pocket the difference in price.
In the case of a double square, I just ordered a 4 inch double square to replace the one that has hidden itself somewhere. I ordered an iGauging square for less than $20. It doesn't have the sliding stop. I can keep the blade and fence flat myself for the extra $50 difference.