22 Rifle Single Shot

JC Higgins model 103.18    22  / Marlin 100 and 100g  Dovetail Size 3/8"
2.25" plus inches long

A standard 3/8" (9.525) dovetail slot refers to the dovetail groove used on rifles for mounting sights and scopes. The groove has a 45-degree angle, with the 3/8" (9.5 mm) dimension measuring the bottom width of the slot. The top width of a 3/8" dovetail is approximately 12.7 mm. 
Key features of a 3/8" dovetail slot:



30713-103.18$27.95

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Here's what the search results indicate:

In conclusion, while 3/8" is a prevalent size for rifle rear sight dovetails, it is not universal. Different manufacturers and even different models from the same manufacturer may use other dimensions.

To determine the dovetail size for a specific rifle, it's best to either measure it directly or consult the rifle's specifications or documentation.