From the Mercedes Service Manual (Model 107 Body & Chassis):
Rims with a hump at outside only are carrying the designation "H". At high air loss of tire, the hump prevents sliding of tire bead into well-base rim, that is, a sudden venting of tire. For tubeless tires at least one hump on rim outer side is specified.
For 6J x 15 H2:
6 - Rim width in inches
J - Code letter for height and contour of rim flange
x - Designation for one-piece drop base rim
15 - Rim diameter in inches
H2 - Rim section outside and inside with hump shoulder
Rim offset - ET
distance from rim center to contact surface of wheel disk, measured in mm
In other words, the larger the wheel offset, the further the inside edge of the rim is from the face of the hub. I don't think it's a problem to use a wheel with a smaller offset than your stock wheels, but using one with a larger offset risks some type of damage (or rubbing), particularly the parking brake. Since your stock wheels are 49mm offset (ET49), using a 48mm (or even a 34mm) offset rim shouldn't be a problem at all. I think the question is, will the 6 1/2" width cause you a problem? That's going to make the rims go 1/4" - 1mm further in than your stock wheels. I don't know if that will be a problem with your vehicle.
1988 560SL Black/Palomino
1988 300SEL Black Pearl/Burgandy
1984 500SEC Anthracite Grey/Palomino