The original Sennheiser 421 was noted as the MD421U...this mic was made of metal and was imported to the States in the 70's and 80's. They had garnered a great reputation for drum mic'ing (kick and toms) with their 5-step roll-off (LF) selector. BUT they also were notorious for being a rather large target for the errant drumstick, and they didn't hold up very well to that abuse. So
Sennheiser "re-did" the 421 a while back, calling it the "II". They make the body out of some sort of polycarbonate that can take the
beating better, and I believe that the "II" is a bit smaller, making it less likely to be smacked. The shock-absorption capabilities are a good deal better,as well, so that the cartridge inside is less likely to be jolted "out of phase". This results in a "nasally" sound quality that was a problem with the originals when they got dropped or smacked around. These mics are also popular in European broadcasting studios, and make a darned good mic on an Ampeg SVT or a Marshall stack.

They also "improved" the frequency response in the higher registers with the "II"s. I perceive them as a little brighter sounding (without being edgey IMHO)

Go over to gearslutz.com and do a search for 421 and AudioGaff. You will find a post I made from a reply to the same question written by a Sennhieser sales rep.

i cant find that post.

Guess, I'll just repost it here...

Greg Beebe at Sennheiser wrote a while ago in response to this question. He said:
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"There are four significant differences between the MD421-U and MD421 II.

Easier positioning:
-The metal inner chassis has better distribution of weight.
-The bass roll-off is contour fitted to the housing making the mic shorter and more "stylish"

More durable:
-Housing is made of polyacetals: a glass composite
-Basket is hardened stainless steel

More immune to the elements:
-The acoustic components are enclosed in the inner chassis making the mic less sensitive to dusts and humidity.

Easier Servicing:
-Individual components can easily be replaced
-Self-sealing acoustic connections: no adhesive or sealing compounds.

The MD421 II does sound different. It has improved transparency, thus a more natural sound. We changed because of improved technologies. New materials and production techniques which enable us to manufacture at even closer tolerances.

Coincidentally, this was not the first time we changed the MD421. We are constantly striving to improve our products. Over the years, the MD421 has seen upgrades in the housing, output connector, basket and capsule assembly.

In my personal opinion, the MD421 II has been accepted, for the most part, as the replacement for the original. However, as you know, there are die hards who swear by the original. Interesting enough, the biggest complaint on the original was the clip. I've not receive one complaint about the clip since the redesign. I attribute this to the mic being more balanced. Now the clip pivots on the mic's fulcrum point."