N.H. Rev. Stat. Ann. § 478:11

If the name of the person whose property or estate is conveyed in either of the cases aforesaid does not appear, or if the name of the original mortgagor or mortgagee of property foreclosed does not appear, the grantors or mortgagors shall be referred to as unknown in the index of grantors under the letter U, and the original mortgagee in the same manner in the index of grantees.