The owners of the former Plymouth Rubber property may have been dealt another blow as selectmen opposed a 105-bed assisted-living facility on the site. In a hand-delivered letter to the Zoning Board, selectmen said the Revere Street senior living proposal could jeopardize a similar project on Turnpike Street. Selectmen also cited traffic, financial, and environmental concerns. After receiving the letter at its Oct. 17 meeting, the Zoning Board delayed a decision about granting a special permit to the project. Attorney Paul Schneiders, who represents site owner Canton Holdings, said at the meeting he had never seen selectmen intercede on behalf of one project over another, and urged the Zoning Board to approve the special permit. The Zoning Board plans to discuss the matter again on Nov. 14.