The aluminum H-D rims are all made by Borrani and first appeared in 1960. There are several different types, '60-'63 front in 18 or 19, '64-'69 front in 18 or 19, '60 -'69 rear in 18. Any of these interchange with the steel rims for the same type brake. The '60 and '64 types do not interchange with each other, the spoke drilling is different. In '70 the rims changed to a deep drop center type, much stronger but heavier. The spokes on the deep ones are shorter so a pre'70 rear and '70 up rear do not interchange unless spokes are also changed. The front also changed again with the arrival of the disc brakes. Once again they do not interchange with the drum brake type and no rim was made for the '73 style hub, only the '74 and later. All the original equipment rims have the part number stamped in them near the valve stem hole. There are no markings on the outside like normal Borrani rims. There are also several variations that are race bike models (different numbers) and even a few for the US made lightweights (identifiable by having only 36 spokes)! Normal tires fit them all.
There is one other difference between the pre '70 rims and the '70 and newer aluminum rims and that's the bead seat profile. The earlier ones are all WM profile. The '70's ones are CM profile.
The CM profile rims are made for use with Goodyear Taper Bead Base tires. I think it was an attempt on the part of Harley and Goodyear to corner a part of the motorcycle tire market.
It doesn't mean you can't use them. I have an 18" steel CM profile rim on the back of my '96 Sportster, with a Continental SuperTwin 120/90-18 tire. And am about to lace up a 19" disc brake front aluminum rim to a mid star hub to use on the back of my Pan project with a 4.00 x 19 Sears copy of a Pirelli MT53.
The rear Sportster 18" rims as well as both styles of the front 19" rims are 2.50 at the bead seat, so the width is OK to use a front on the rear. You want to use the one for the disc brake though, because it's drilled for a smaller diameter hub. The 18" will be part number 43008-70, and the 19" will be part number 42998-74.
Here's a link to a tech piece that has more details.
http://www.jockeyjournal.com/forum/showthread.php?t=22085