Clayton, I just found the neoprene covers and as you say, they're sized S,M,L with length and width dimensions clearly given. Seems like the nylon covers, check REI for example, give sizes in those crazy numbers with no explanation of what they mean.
Just ordered a medium NRS.
Thanks for the help!

: Hi Ian,
: I use a neoprene cover they fit the best. I like Snapdragon but any
: one's cover will do. A size M or medium NRS neoprene cover will
: fit a Spring Run coaming.
: Most size charts give 2 dimension limits, one for length and the
: other for width. This is measured at the outer edge of the
: coaming lip.
: I hope this helps to clarify.
: Clayton Plunkett