As far as I know, MK1s are serials starting with "700x" which later became "SZ1". There's the Japanese domestic champagne/brown and the US market silver/black. The MK2/A is "SZ2" which is silver/black in Japan and all black in the US. Keep in mind that Stax calls it's domestic amps "A" and export amps "II". I think that's where people might be confusing differences between MK2 versions where there aren't any. The so called "MK2.5" is serials "SZ3". According to Spritzer the tonal balance was shifted quite a bit on SZ3 versions which are supposed to be considerably brighter.

I've never heard of a silver / black mk1. To my knowledge, the ones for both Japan and the US were champaign with brown leather, and the ones for Europe were champaign with black leather (aka "007BL". The silver with black leather is the Japanese mk2 (aka "007A") and the all black (aka "007mk2") is the US version.

I was talking about the 007mk2 versus 007A versions of the SZ2 specifically. Some people insist there's a difference: ie. Stax released the 007mk2 first in export markets and then based on impressions of it tuned the 007A slightly for their domestic market. I've heard stories about the bass being better on the 007A, on it sounding more like the mk1, etc. Sometimes the very same individuals saying one thing were saying completely different, contradictory things prior. Personally I don't recall hearing a difference between the 007A and mk2.

I haven't heard the SZ3 yet, but I've seen the same range of claims about it.

Personally I take these kinds of things with a grain of pepper.