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Actually the 18, falls within 21A's weight tolerance.
The rod weight for the 18, runs 477 - 481G
For the 21A its, 474.5 - 486.5G
All the small end weights are nearly the same but, the 21A has a larger tolerance.
The 48 rod, is lighter though at 462 - 468G. The weight difference on it, I believe is in the cap, itself.
It's a little hard to read but, this is the actual Ford spec for those rods.
I don't have the weight info for the 91 rod though, it appears that any increase in it's weight, would be do to weight of the cap, itself.