Quoted:
Can someone explain the difference between the dedicated, top, and bottom 9mm Hahn adapters?
bottom - slides in from the bottom of the magwell - has a built in actuator that works in conjunction with the std. bolt catch to provide LRBHO - get this if you want to switch between 9mm and other calibers quickly and retain last round bolt hold open LRBHO
top - slides in from the top of the magwell so it can't ever fall out - has no actuator so when used in conjunction with a std. bolt catch, there is no LRBHO - you can use this block and still switch between 9mm and other calibers fairly quickly
dedicated - kind of a cross between the top and bottom blocks in that it is a top load with a built-in actuator - you must remove the std. bolt catch to install/remove the block and it has LRBHO capability - not the ideal solution if you intend to switch calibers much
Note - you can use the top loader with a 9mm bolt catch which effectively turns it into a dedicated unit with LRBHO - the advantage over the dedicated unit is that it is a little cheaper to go this route and you have less moving parts to worry about - the follower acts directly on the 9mm bolt catch instead of having to push up on the built-in actuator which then pushes up on the std. bolt catch for LRBHO - older blocks had tighter tolerances and sometimes the actuator could bind when dirty and not activate the bolt catch, Hahn has supposedly changed their tolerances to reduce the likelihood of that happening but I can't say whether RRA or others have also