I have an open overcharge case with DHCR going back several years now.
The rent history for my unit shows *EXEMPT APARTMENT - REG NOT REQUIRED* for a 7 years after which the rent jumped more than $1100.
I've tried to establish who had previously lived in the unit using a variety of methods, and today I searched public records at housing court and found a case in which my landlord brought a previous tenant in the unit for nonpayment of rent -- in the middle of the span during which no registration was reported to DHCR.
I'm not sure how this will bolster my case with DHCR, but a note for the more informed tenants here that this is an option to further research your unit and it's history.