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J Levinethis one is easy for me...over the last seven years I have seen fewer than 10 drum backs sold at auction or purchase (one, the bridwell, has been sold four seperate times[counted only once]). There have been nearly 4 times as many hindu sold...might be because drum is tough in t-206 so hoarded in the t-205 set. Personally, I have owned 3 hindus in 10 years (still own two) and have never owned a drum.
Kevin Struss and Lew Lipset (among others) have said that drum is scarcer. This might be due to the fact that hindu was virtually unknown until the late '70s. A large hoard was found then and since then a few other stashes have popped up of hindu. this has yet to happen for drum but I am hopeful 
-Joshua
PS My word is not gospel and I do not speak for other collectors but this is my opinion based on my research.