From semi-contemporary notes at the (seemingly inactive and archived) bikepro.com site: (circa 1996?)
"In 1992, Shimano released their sealed bearing cartridge bottom brackets for use with their road and mountain bike parts groups, this is the "UN" series of bottom brackets. The three, and the added Dura-Ace 7410 model, have some variations in their appearance and weights. The quality differences in terms of materials used, tolerances manufactured to or bearings used are less discernible.
We called Shimano headquarters in Irvine, California to ask how they differ. Robert explained that the more expensive models are made of better steel and bearings."
Further info was apparently unavailable.
The site also describes their findings from tearing down some of the BBs:
"The UN-50, when you removed the outer rubberized seals around an aluminum washer, revealed seven approximately 4.5mm diameter steel balls in a plastic retainer. With both outer seals removed, you can look straight through the B/B unit. These outer plastic seals, and their integrity, ... are all that keeps the outer world shielded from the inner 7 steel balls. The UN-51, UN-71, UN-91 ... B/B all outwardly appear to have the same bearings as the UN-50 we tore apart for inspection."
Regarding other differences:
"The UN-51 is the successor to the UN-50 and least expensive of the UN bottom brackets. One obvious improvement Shimano made to the UN-51 was to upgrade the left bearing cup from a grey plastic part, which stripped and cracked, to a stamped and machined steel part. The central core of the "sealed" unit, which retains the bearings, is made of fairly sturdy steel. The exterior of this central core is smoothly turned, with the right side bearing cup pressed permanently onto the right side of the core. The right cup is made of machined steel that is colored Silver.
The spindle on the UN-51 appears to be made of cast Black steel that is turned on a lathe to perfect the outer dimension. It is solid, with the ends bored and tapped for the crank bolts to screw in. The left bearing cup is made of Silver colored steel, ..."
"The UN-71 is the successor to the UN-70, and is the specified Shimano sealed unit B/B for use with the Ultegra Road group and the Deore XT Mountain group. The core is made of steel with the area between the cups turned on a lathe to reduce the diameter and thereby the weight of the unit. The right bearing cup is made of Black colored aluminum and pressed permanently onto the right side of the unit's central core.
The spindle is made of hollow Silver colored steel, (appears to be the same one used in the UN-91), with the ends drilled and tapped for the crank bolts. The left bearing cup is made of aluminum,..."
"The UN-91 is the successor to the UN-90 and made specifically for the XTR and Dura-Ace crankset, because the B/B's shorter spindle length and of how deeply the arms recess over the spindle toward the bottom bracket shell. Shimano suggests that combinations other than these cranks are not possible for this B/B. The center core of the UN-91 is made of steel. Between the bearing cups, the core is turned on a lathe to reduce the diameter, and the weight of the unit. The right bearing cup is made of Silver anodized aluminum and pressed permanently onto the right side of the center core.
The spindle is hollow, made of a silver colored steel, like the UN-71, with the ends tapped for the crank bolt threads. The spindle appears to be cast because of the appearance of it's 2 degree tapered sides, and is then machined after to perfect the surfaces. The left bearing cup is made of machined aluminum... "