I only know a few hundred Co-ops in Queens and almost all of them have Offering Plan (prospectus), with very few exceptions. As for Proprietary Lease, let us not confuse that with land lease or ground lease. Proprietary Lease is a must for a Shareholder (tenant) to be able to live in a Co-op (landlord). When you "buy" a Co-op unit, you actually buy certain number of stocks which is allocated to a particular unit for which you get a Proprietary Lease and yes, it does have an expiration date which, Co-op will keep extending to the eternity. For legal purpose, you are a tenant in a Co-op and if you violate the Proprietary Lease, you could be evicted even though you "bought" the unit.