One more caveat to consider--and, forgive me, I don't know the answer: There *may* also be some differences in the width at the of base of the chainring where it meets the spider, as viewed from the drive side. I'm not talking about thickness when viewed from above. It may be that the spider arms are not the same size/width where they meet the chainring. (I hope I'm explaining this clearly.)

I mention this because I am also in a similar situation, in that I bought a used 7400 crankset which came with replacement chainrings (Shimano and Specialized). The small chainring is 42T, and I plan to replace with 39T at some point. I sampled a couple spare 39T Shimano chainrings with the same BCD (Shimano 105s, if I remember correctly), and the chainrings did not line up flush with the edge of the spider arms. I expect the chainrings will function fine, but may be objectionable to a persnickety aesthete.

I would love for someone with a 7400 series crankset and, say, 7700 series crankset to be able to confirm.