1. if X admitted that the property purchased by him was infact a HUF property, then upon his death, his undivided share in the HUF property would devolve on his legal heirs as per s.8 of Hindu Succession Act
2. further the male descendants of X [ie his sons, grandsons, greatgrandsons] would have their own independent share in the HUF property the moment they are born
3. thus when the male heirs of X release their right share and interest in the HUF property in favour of the firm/company, they cease to be coparcenors of the HUF property
4. however whether the undivided shares of X's heirs are duly transferred by release to the firm, needs to be ascertained from a registered release deed
5. also if there was no consideration passing from the firm to the heirs of X, then the release will be considered as a gift and a gift is also required to be effected by a registered document
6. further one of the essentials of a valid gift is acceptance of the subject matter of gift by the donee. In your case [assuming no consideration passed from firm to X's heirs and therefore transaction is a gift], since the releasee has not signed the document, the gift is not complete