Ron Guth: In 1956, the Mint used two different reverses to strike Proof Half Dollars. The main difference is visible in the small eagle on the reverse and relates to the number of feathers that are visible to the left of the perch on which the eagle sits. Type I shows four, clear feathers to the left of the perch; Type 2 shows three feathers. In 1956, the Type 1 reverse was used on both Proofs and circulation strikes, and the Type 2 reverse was used only on Proofs. In subsequent years, all the way until the end of the Franklin design type in 1963, the two reverses were used intermittently, but the Type II is most commonly seen on Proofs.
Among the 1956 Half Dollar Proofs, the Type 1 is much less common than the Type 2. The PCGS Population Report (as of February 2012) shows 885 Type 1s and 8,587 Type 2s. This rough 1:10 ratio is slightly higher than Breen's guess-timate of 1:20 (5%), which was actually not too far from the mark.
Cameo Proofs are about five times as rare as the non-Cameos. Deep Cameo Proofs are exceedingly rare.