There are differences in the Ultrabay adapters (they are NOT caddies!) for the T400/T500 series notebooks. First, they use the Ultrabay III adapter, which can be iodentified by the ejection tip on the rear. Here it is compared to the old UB II used in the T40/T60 series:

Within the UB III family, there are currently two adapter sizes in terms of thickness.  First, the original UB III had a 9.5mm thick faceplate.  This left a small gap in the laptop when fully seated:

The newest UB III adapter has a 12.7mm faceplate and seats with no gap:

A major difference, however, is that a drive in the 12.7mm model is fastened by 4 screws in the back, and cannot be mounted in the standard Lenovo drive caddy.  So, what is the Lenovo drive caddy?  It is the perforated metal cage equipped with soft rubber rails, standard in most Thinkpads.