The privately held firm Symbolics which holds the remaining assets and intellectual property of the former lisp machine manufacturer Symbolics, Inc is reported on the US Government's spending disclosure site to have quadrupled its revenue over last year. The transaction is reported as being for:
Maintenance, Repair and Rebuilding of Equipment: Automatic Data Processing Equipment (Including Firmware), Software, Supplies and Support Equipment
This comes in the form of a 225,000 United States dollar award as a primary contractor. The dramatic increase in the contract award along with the steep increase in the price of Symbolics hardware on resale markets like EBay suggests that over the next few years working lisp machine hardware may become unavailable at any price normally associated with computing hardware.
Have you considered asking David K. Schmidt for an interview?
AFAIK he isn't bound by any secrecy oaths; perhaps he (or possibly even the reclusive Prof. J. C. Mallery, thought to be the current owner of the SMBX copyrights) would be willing to add something to the (presently – very murky, almost UFOlogical) public record…
This would make an interesting follow up.
And be sure to ask Kalman Reti, the last developer.