What you have there is an Alembic Series I LSG (Long Scale Guitar) with an Omega cut. If the serial number is under 20, it is either a ‘72 or an early ‘73. As for value, Alembics do seem to have increased in value during the pandemic. You can check out sales on eBay and Reverb.com. Make sure to look at the sold prices as opposed to the asking prices, there can be a significant difference. Ballpark, I imagine you could get somewhere in the $5000-$8000 range.
Bill, tgo