The earliest depth adjustment mechanisms were manual, i.e. just like wooden planes the depth adjustment mechanism involved manually lowering or raising the blade and then tightening the lever cap. Adjusting the depth of the blade in these block planes may involve using a plane hammer, in a similar way to how wooden plane blades are adjusted. The difference between wooden and metal block planes is that the metal ones usually employ a lever cap with some form of clamp screw. These clamp screws were historically very rudimentary, but as time progressed, they graduated to a more refined, machined form. Below is a Sargent No.107 from their 1894 catalog. This is an extremely rudimentary plane, and is an indication of the lower cost of the plane ($0.60 versus the No.1307 which had depth, and lateral blade adjustment, and throat adjustment mechanisms, and sold for $1.75).
This simple form of blade adjustment, i.e. none, remained important in lower cost planes, and planes that were too small to accommodate a depth adjustment mechanism. Below are two examples of manual blade depth adjustment. On the left is the Hobbies No.10, with the blade resting on the butte attached to the plane body. On the right is a Pexto with similar set-up. The Hobbies clamps the blade down using a butterfly nut attached to the lever set screw. The Pexto uses a more traditional clamp screw.
Below are two examples showing the diversity in circular clamp screws. On the left is a simple clamping screw from a UTIL aluminum block plane, with very large lugs on the side of the wheel. On the right is a more refined Record No.102 with a wheel that has fine linear knurling. The smaller clamp screw has the added advantage of sitting under the lever cap, and is less prone to accidental movement when being used.