[uncle max]

The only surefire way I know to spot a 40 is to remove the airhorn and examine the idle transfer channels, or lack thereof.

Originally Posted by Bruce Lancaster

Well, nothing is a safe call at 10 feet because lots of the differeces interchange and many elderly carbs are mixtures.
A relatively big difference visible on the one up above is the right side linkage...a 97 choke lever would have a tab at top rear with a little hole in it and the casting would have a matching boss to hold the springloaded widget that secures choke in off position. Naturally, many free-range 48's now have 97 parts there...
Now, spot a 40 in a pile of 48's...