The bases have a slight concavity which allows mounting on either a round or an octagonal barrel. Height measured from a flat surface and will sit .005" to .018" lower on a round barrel.

Lyman/Unertl crescent on one side and Winchester dimple on the other.

Hole spacings for scope blocks are mostly standardized. Since about 1920 or so, they seem to have been set at .560" front and .860" rear, with holes tapped 6-48. Sometimes the .560" is .562", realistically that .002" difference is not significant. There are numerous exceptions to these rules. Older original Stevens rifles (pre-Model 414 rifles) were often drilled at .700" front and .825" rear, with 5-40 holes. Rifles drilled for Weaver-type mounts have holes spaced at either .500" or .504". Remington 40X and 700-series rifles have a spacing on the rear ring of .600". Remington 513T factory holes are not 6-48; they are 6-44's. Kimber 82G has a front sight (at the muzzle) hole spacing of .500".

Multiple heights available. Also available: 3/16” with 6-44 screw for Remington 513T.