The early transom mounts did not have a resovior bottle, hence no fitting for it. The newer (90 or so) drives will bolt up without leaking. But you cant put an 88 drive on a new transom because of the fitting on the helmet. There may be a recess on the older drive, but I am not sure.. I do know the bottle will be non functional. Besides you dont want the old stuff anyway..

So from about 90 up they will all fit. When changing to XR with old style ujoints, the gimbal bearing must be replaced along with spacers added to the trim cylinders to limit travel in the up position so it doesn't pinch the yoke on the ujoints and break it.
HD gimbal uses a differnt helmet and gimbal ring using a different hinge pin setup. All the bravos will bolt to either the standard or HP gimbal.
B1 drives with serial number prefix of OL or older have thin floored helical cut upper gears. OM and above (2000, I believe) have new X gears. Same gear just with thicker floor in the hub. Inner bearings are different height, so race on the tower is shorter. XZ drive uses the X helical cut gears, but larger pinion gear input shaft (same size as XR gears)
XR drive used straight cut, net forged upper gears. Also uses the 1 1/4" propshaft. XR is suppose to be the stronger setup, but there has been a problem with teeth breaking off the driven gear at the root.. Never saw that 3-4 yrs ago.
Before the X gears came along, Merc did some bead blasting on the upper gears and called the Diesel gears and they were used in the drives that were put behind the Diesel engines. They proved to be no better than the regular gears.
B2 is dual prop and is for heavy boats not pleasure boats.
B3 is dual prop for getting on plane quicker, with low hp. And max speed is below 60mph I believe..
Top cap is important when LH rotation is used. I like to use the XR ribbed top cap, even on RH rotation because it uses a steel tower in the cap. But on RH all the pressure is on the lower gear in the case.
Top caps must be measured for race depth to match up with the cap being replaced and any difference is compensated by changing the thickness of the race.
Hope this helps..
Dick