Well, it's a swimmer nymph, which includes Baetis and Iso's. Fairly similar body shape between the two, and they get "fatter" (which would mean more uniform) as they get older and go through various instars.
As for coloration, it varies a lot based on which instar. Most Baetis nymphs I've seen have 2 lines, or dots. Yours has one broken line. But in addition to which instar, there are sooooo many Baetis species I hate to base it on that.
Time of year, you have a point. More likely to see Baetis right now. Though obviously ISO's are down there, just much farther from emergence and less active. My comment was more based on size. You indicated it was a 14ish. I'm not sure I know of any Baetis that get much bigger than an 18, maybe a 16 in some waters. That's the only thing that gave me pause to say Baetis. Where as for an ISO, that's rather small, which makes sense since he still would have a lot of growing to do...
So super big Baetis, fully mature and ready to pop, larger than most specimens? Or immature ISO? Or something else? I don't know. It's a swimmer nymph, confident in that! lol.