I have three very popular LGS's near me. I see an awful lot of used Ruger MKIII's & they're going for very low prices. Rarely do I see any used MKI's & MKII's & I have never seen any used MKIV's. I have two MKIV's that I purchased recently that I'm absolutely thrilled with. Is the MKIII a undesirable model? Some of the prices I've seen for the used III's are quite tempting, but I'm concerned as to why.