Investment condition for tax-exempt trusts clarified: timing and treatment of certain asset holdings and debenture income for exemption. The amendment adds a temporal investment condition: assets not held in the prescribed forms are subject to a one year post acquisition holding rule or a later cut off date to determine their treatment for exemption. A distinct provision protects exemption for income other than income from certain debentures acquired in a prior window, so long as funds cease to be invested in those debentures by the prescribed cut off; both changes are made with retrospective effect.