Hi, Keith,
Check out Reverb, serial number 76-399 is up for auction. It is a purpleheart cap and back with a walnut core, neck through built of birch and walnut stringers. I've been eyeing this bass for a bit, but the scratch to buy it, and fix another, is not yet in hand, so check it out. The purpleheart has aged really beautifully, in my opinion (
https://reverb.com/item/11645443-vintage-medium-scale-alembic-series-1). There was a build request for this bass that was posted to the forum ten years ago:
http://club.alembic.com/index.php?topic=6325.msg67543#msg67543.
Reverb also has a more expensive medium scale series 1 from 1978 for $8K with a really complete package from the mother ship (listed as serial number "78-12187", but I read it as "78-1287", which makes a lot more sense given 76-399, above). It's claimed to have mahogany stringers in the neck, and has this great halcyon and burnish appearance. I am not sure about the price, but it, too, is a beauty. I didn't see a build history already listed here on the forum. The surfaces are a bit checked, so I am a little confused why this bass will be worth $8K unless someone pays that (re: "what the market will bear").
Reverb also has a shorty in burl walnut (
https://reverb.com/item/9702095-alembic-short-scale-bass-series-1-1979-burl-walnut) for $6,316.27, and a long scale '77 series one, serial number 77-840, in "natural" for $6,250 (
https://reverb.com/item/12252164-alembic-series-1-long-scale-1977-natural-superb-condition). It's for sale by the original owner who now has the hots for a Zon 6 and wants to sell this puppy. All original build records and some of the goodies are available. I didn't see an existing build request on the forum for this bass, but picture of the build records from Alembic show a schedua top and back, a mahogany core, the neck is maple with purpleheart stringers, ebony fingerboard, and dark vermillion veneer.
Searching only for "alembic series" pulls in a few other basses, but they are over the $9K mark, and I assume that some of the ones above may be less than feasible given your stated parameters.
eBay has this neat medium scale omega cut standard point from 1975 for $6,950 with the predecessor to the diagonal adjustment pickups, namely the "tab" pickups that were phased out shortly thereafter (
https://www.ebay.com/itm/ALEMBIC-75-Series1-Omega-Cut-1-1975-good-condition/332658741829?hash=item4d74019245:g:GucAAOSwWDBbAgrW).
The 'Bay also has a long scale '81 that ends in 2 hours (it's 6:56 PST, as of now):
https://www.ebay.com/itm/1981-Alembic-Series-1-Bass-Guitar/123134941643?hash=item1cab6a19cb:g:NFEAAOSwSY9a51fT.
And a short-scale '79 series 1 for $6,830:
https://www.ebay.com/itm/ALEMBIC-SERIES-1-SSB-1979-Used-Electric-Bass-FREE-Shipping/173327878276?epid=5018221222&hash=item285b258884:g:ZHAAAOSwDvVa18ziAnd another short scale, this one from '77 with an interesting top for $6,445.00:
https://www.ebay.com/itm/1977-ALEMBIC-SSB-Series-I-Electric-Bass-Excellent-Condition-Vintage-Rare-wOHC-B5/332635524106?hash=item4d729f4c0a:g:3~EAAOSwM2RZgtbJ.
This '77, unknown scale length, is pick up only in Manhattan or Yonkers, NY for $5,995.00:
https://www.ebay.com/itm/Alembic-Series-1-bass-guitar-1977-vintage-excellent-condition/332631716799?hash=item4d726533bf:g:aZgAAOSwZtJW-GS1.
Here is a medium scale 1980 series 1 (walnut/mahogany body, maple and walnut stringer neck) for $5,400:
https://www.ebay.com/p/Alembic-Electric-Bass-MSB-1980/2161372089?iid=173318862221&_trkparms=aid%3D555018%26algo%3DPL.SIM%26ao%3D2%26asc%3D50999%26meid%3D569e376f690943078d53c7bc849dbb7e%26pid%3D100005%26rk%3D2%26rkt%3D12%26sd%3D173327878276%26itm%3D173318862221&_trksid=p2047675.c100005.m1851.
And I am sure, despite all of the options not being exactly what you were looking for, there is most certainly a lot more out there to be found. Good luck!