Perhaps not much of a mystery. Think it might be a Jamis of some sort (or possibly a Novara or Diamondback), repainted, ridden hard, put away wet and left to molder. If so maybe someone can help with the model?
Serial number is hard to make out under the paint, but appears to be an "A" followed by 4 or 5 indecipherable numbers (which, along with the seat tube lug, led me to Jamis).
Other notable features include double water bottle braze ons on the downtube, fastback seat stays, plus attachment points for a portage strap. Fork dropout/lugs look an awful lot like those I've seen on old Ritchey Logics. Tange stamps on the fork ends.
​The jumble of high end components caught my eye initially, though most are long past their prime. And the paint itself has a nice shimmer where the rust hasn't crept in.
... Should be a fine winter commuter project regardless of what it is, but appreciate any identification help y'all might be able to offer!

I believe the cranks are Cook Bros Racing (CRB) from early 90s. I flipped a set I found on a similar condition Klein I found at a GW out in the sticks.

I like the lugged fork dropouts.... and pieced together headset. That seatcluster is close to some Japanese Novaras I've seen, but they had a different canti brace as I recall. Are those Suntour Butterfly shifters without their plastic trim?

The brake bridge looks like a Scott MTB I have but now that I go and look at it, the stays of mine join the seat tube below the seatpost binder bolts, not as part of it. Looks like a nice frame though, I love lugged MTBs.

Thanks for the input folks.

I believe Dylansbob is right about those being Cook Bros cranks - there's a crack on the NDS arm unfortunately, which I understand is/was rather common. ​​​​​​
Pretty sure those are Mavic Wishbone/Y shifters, and are meant to be mounted under the bar (the cable exit angle sure seems awkward in the current setup).

If I'm interpreting T-Mar's serial number guide correctly, it looks like Scott frames were being sourced from the same builder/factories as Jamis & Novara at the time, so all these seem like viable possibilities. The specific combination of that pointed lug/fast-back seat cluster with TIG-welds everywhere else seems a bit elusive, Google image search-wise at least. Any other telltale indicators I might be overlooking?
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I'm now convinced this is actually an Ibis Avion frame, c.1988-89, right around the time when they were offering customized braze-on options, and just before ibis stopped sourcing frames from Japan for this model.