Have a single lever mount on a 99:http://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=187553&page=1
tried several double lever brackets on it, both early and late ... the early brackets have rounded end levers, the late ones are squared off. A few went on easily, most stuck partway.
One was loose. It seems clear to me that the brackets were hand fitted to the dovetail bases. Do not know if G&H does this today. Also tried a late Jaeger bracket. Did not go, but almost did. I think a little hand fitting would slide it on. Also of note, I have one extra single lever bracket, waiting for the next G&H rifle that shows up needing one. This also was recalcitrant about going onto the 99 single lever base. From all this I deduce that the "official" specifications for dovetail dimensions for both single and double lever mounts are the same, but that most of them require hand fettling.