For a set P of n points in the plane R^2, let A(P) be the set of multiplicities of P, i.e. the sizes |ell cap P| >= 2 of the subsets of P lying on a common line ell, and let F(n) be the number of distinct multiplicity sets A(P) as P ranges over all n-point sets. Erdos problem 607 (erdosproblems.com/607) asks whether F(n) <= exp(O(sqrt n)), remarking that the sqrt(n) scale is easy to see to be best possible. We prove the upper bound F(n) <= exp(c sqrt n) for an absolute constant c > 0. The proof is a three-step reduction to a published result of Colbourn-Phelps-Rodl, "Block sizes in pairwise balanced designs" (Canad. Math. Bull. 27(3), 375-380, 1984): (1) the lines determined by P, restricted to P, form a pairwise balanced design (PBD) of order n - every unordered pair of points lies in exactly one such block; (2) the profile set of block sizes of this PBD is exactly A(P); (3) Theorem 2.4 of Colbourn-Phelps-Rodl bounds the number g(n) of profile sets of order-n PBDs by exp(c2 sqrt n), hence F(n) <= g(n) < exp(c2 sqrt n). Combined with the grid construction of Szemeredi-Trotter (1983) this settles the order of magnitude exactly: F(n) = exp(Theta(sqrt n)). The essential observation is that plane line collections are precisely PBDs, so the published design-theoretic bound applies to multiplicity sets directly. A Lean 4 machine-checked formalization of the reduction is in progress for the Palomar registry and will be linked from this deposit upon acceptance.