Donations to assist the approximately 60 residents of 10 units at 1469-1471 Beacon St. that were destroyed by fire Monday are being handled by the Brookline Municipal Credit Union. The tax-deductible donations should be made out to the Beacon Street Fire Fund and mailed to the credit union at 334 Washington St., Brookline, 02447. Fire Chief Paul Ford told selectmen at the board’s meeting Tuesday that the fire started in a basement electrical closet, and sped through the building’s open, wooden staircases and along pipes cut through floors. The cause, while not of suspicious origin, had not been determined.