"does the IRS leave you alone if my earnest guess is close 'nuff?"

Depends on the agent.  Your options are to get returns as far back as you can and reconstruct assets to the best of your ability or to not take any depreciation because you don't have any backup.  You need to discuss the issue with the client and let them decide.  After they think about it for a millisecond and select option 1, remind them there is exposure and you probably want to get something in writing -------------------- just in case you don't get the "friendly" agent if the return is ever audited.