from what i can tell the modern and current model is the mkII which has a five position, plastic collar bass frequency control at the point the cable attaches to the mic. previous models didn't have this. if there is any other difference, i don't know.

The 421 has always had the variable low-frequency roll off filter, at least since the mid-1970s. It's not exclusive to their newer model. I have a couple of those microphones and they are completely bitchin'!!! They are really fabulous on snare, bass drum and all the toms. The NBC FM radio station I used to work for used them for their announcers but I never really cared for them on voice, spoken or singing.

I can't imagine that the newer update model is really anything more than a neodmyium magnet upgrade? It's a classic that has been around since the 1950s. They are quite consistent. They sound like an SM58 on steroids, only better!

My favorite bass drum snare drum mic mmmmmmmmm........
Ms. Remy Ann David

The 421 has indeed always had the 5-position voice-to-music switch. The difference between the MD421U5 and the newer MkII is the cartridge with the new magnet (re:what Remy said), and the shell is now a tad bit smaller (to get into a drumkit a bit easier) and is made of some polycarbonate material instead of the metal they used to made from. Sennheiser said that they did this so that the mic would hold up better when bashed by the errant drumstick. On earlier models, that metal body would "telegraph" the impact right to the cartridge, quite often knocking it out of comission, giving it a very nasally tone.The newer material absorbs the shock and protects the cartridge. They all sound great on horns and percussion, don't they?

Greg Beebe at Sennheiser wrote a while ago in response to this question. He said:

"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."

great to know. sorry for the wrong impression.