Basically the same as the john guest type fittings. The tubing seems small to me but it depends on what you use it for i guess. You can find them on aliexpress for cheap if you dont mind waiting on shipping. Williams is obviously better if you think you might return it. Micromatic sells similar parts as well.
Was told these are pretty common/popular in uk/auz/nz.
Its actually smaller i think. 3/16 and 1/4 inside i believe. Those types of fittings always call out the outside diameter, not inside like we usually do. Depending on thickness of material the id can vary widely. You really gotta pay attention to the sizes and how they interact.