Originally Posted by Crescent Cycle

I get my quill stems from the junk bin, but Origin 8 makes some cheap ones, and Nitto makes some premium ones. AVA makes killer stems but my AVA is my personal favorite. Vintage ones can be 25.0mm, 25.4mm, 25.8mm, 26.0mm, 26.4mm clamp diameters. Modern ones come in just 26.0mm and the very rare 31.8mm. There's also two different post diameters, 7/8" for 1" steerers, and 1" for 1 1/8" threaded steerers which were used on some threaded headset MTBs. You can also pick up a quill to threadless adapter if you already have a box of threadless stems.

You will notice a lot of vintage small frames come with short stems, 60mm, 70mm and 80mm which are short compared to the modern sped 90mm, 100mm and 110mm. That's because the top tube length didn't vary much at all between sizes. When it did, the majority of the difference usually came from seat tube angle and had little effect on reach. Reach was adjusted using very short stems. I don't find it surprising that you're able to make a vintage small frame fit. Many of them are just medium frames with shorter seat tubes, shorter head tubes and shorter stems. Below 52cm shorter head tubes don't do anything except keep the top tube horizontal with the shortened seat tube unless you cut down the stem height.