The old units have round pushbuttons while the reissues have the same small, square buttons that can be found on most other modern #API gear.
The meters are also a bit different (the lens on one of our old ones is a bit worse for wear).
Sonically, we treat them as interchangeable. It's possible that the old ones have a bit of extra flavor, but we've never really shot them out or investigated too deeply—they all "do the thing," and we tend to just patch in the next one and get on with life.
The possible exception may be if we are using two as a pair—there, the newer ones may get the edge in being reliably similar-to-one-another.
We absolutely love these compressors—they're fantastic on many things, in particular all kinds of guitars (acoustic and electric).
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