The MD 421-N has the Tuchel (screw DIN) connector.
The MD 421-2 has the large Tuchel connector (flat pins)
The MD 421-HL is dual impedance and *unbalanced* and does not have the bass roll-off switch
The MD 421U has an XLR connector
The MD 421U-4 has an XLR connector and is black finish
The MD 421U-5 has an XLR connector and is black finish and the stand clip has the horrible 5/8" American thread.

All these are the original Mk.I version.

Personally I would go for the U or N version - definitely do *not* get the HL version (unless it's *very* cheap).

It is possible to convert an N to a U but you have to replace the complete filter module, so it's not so cheap.

I hope this is clear.