I believe they are measured across the width of the ramp. There's at least two sizes for the 336/30's. Either you have an older design with a 3/8" or the newer type with a 1/4". I'm not sure where Marlin changed over but it it probably after 1981 because my 81 has a 3/8" and my 2000 has a 1/4". you can see that the 1981 ramp is much wider and has both mounting screws exposed :
On Edit:
Here's another picture that addresses the question a little more completely:
Thanks N578MD and TOMRAY for you responses. Wasn't sure if dovetails widths were the same for both. I have a 1/4" ramp (width) and Skinner peep. Think I need a ~0.45" blade. Just sent order to Andy.
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