Oct 25 (Reuters) - Ntsb :National Transportation Safety Board Issues Letter To Tesla'S Elon Musk.Ntsb, In Letter To Musk, Says "Deeply Concerned", That Tesla’S Action To Implement Safety Recommendations Has Not Demonstrated Productivity.Ntsb, In Letter To Musk, Says "Invite You To Complete Action On The Safety Recommendations We Issued To You Four Years Ago".Ntsb, In Letter To Musk, Says Investigations Involving Tesla'S Vehicles Have Shown Potential For Misuse Requires System Design Change To Ensure Safety.Ntsb, In Letter To Musk, Says Tesla Has Still Not Officially Responded To Ntsb Regarding Safety Recommendations H-17-41 And -42.Further Company Coverage: Tsla.O. ((Reuters.Briefs@Thomsonreuters.Com;)).