I checked the TCDS for some other popular helicopters and did not find any specific references to allowing external loads, and no prohibition against them. Just because the 210's T.C. does not permit them, are we to assume they are prohibited? Maybe there is a flight manual supplement?
It is curious to compare the 210 with the 205A-1 or 205B. When looking at them side-by-side on paper, you'd have to wonder why Bell even bothered with the 210? It's even slower than some of it's "twin" brothers (and I don't mean the 212 either).
Certainly a manufacturer can "zero-time" or remanufacture his product. Teledyne Continental does it all the time with their piston engines. Why not Bell?