Crane TP 410 that lists the L/D values for various types of valves.
However by 1976 these values changed. But I've never seen any justification or explanation of the changes.
Here are some examples of changes:
Wedge gate:60s version = 13,76 Version = 8
Swing Check:60s version = 135,76 Version = 50
8" and larger Butterfly:60s version = 40,76 Version = 8" ~ 45, 10-14" ~ 35, 16-24" ~ 25 
The Crange Technical Paper 410 "Flow of Fluids" is based on testing done by the Crane valve company on their valves.
They may well have decided the old values were overly conservative, hence the decrease in equivalent diameters.  It may be that better computing and measuring equipment allowed them to refine their results.
BTW, the values you list are mostly still valid per the 1991 printing that I bought about four years ago. There is a second value (of L/D= 100) listed for a swing check valve, depending on configuration.
In the 70's there was a move to higher capacity valves. The L/D is related to the Cv and the valve recovery factor. You can develop the actual L/D of the valve but it is so dependent on the friction factor of the pipe that it is generally of limited usefulness once the piping design is done.

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