The Mk IV/80 will have the collet-type "accurizer" bushing. That is the principal difference.
Quick recap:
Mk IV Series '70: Started in '70/'71 and added a "Mk. IV" barrel that employed a spring-fingered "collet" barrel bushing for improved accuracy. No other major changes. This barrel/bushing system was on Government Model guns only, not the Commander models. There was no "Series '70" Commander.
Mk IV Series 80: Started in 1983, ran until 1987. Used the same barrel/bushing but added the firing pin safety system.
Series 80: from 1987 on, continued with fp safety and dropped the accurizer barrel/bushing system for the traditional solid type (as used 1912 to 1970). After 1991, the the base model gun was called the "1991/Series 80" to separate it from "enhanced" model guns - now the XSE line.
Confusing, isn't it?
This should make it obvious that "Series '70" referred to the barrel/bushing system and not the firing system/safeties as is commonly confused. Even by Colt's marketing department.
The Mk IV/80 should have a much nicer polish on the slide and flats, not just different roll marks. The one to get is the one that is in better condition and better fit/finished. Most times that is going to be the Mk IV 80. The "current" base model is the 1991 Series 80 Government model. It has matte blue flats and less overall fit/finish than older guns, but mechanically they are very good and perhaps on average better and more consistent in that regard than older guns. Personally, I like collet bushings but they are not worth a $200 difference unless you are a real "old Colt" fan and really like the older guns from a collecting or nostalgia angle. If you just want a gun to shoot, the newer one will be just fine.
Just to be clear, the "current" 1991/80 type gun is the model 02991. I am a little leary as it would be unusual for a Mk IV/80 to be priced $200 over a new gun? But maybe someone is asking a big premium because it is a .38 Super.
