Hedstrom file having a double helix cross-section and a variation of the Unifile is:

Modification of Hedstrom file- Hyflex file. It has a cross-section which presents an “s” shaped instead of the traditional single-helix  teardrop cross-section of the Hedstrom file. Unifiles. They are machined from round stainless steel wire by cutting two superficial grooves to  produce flutes in a double-helix design. They resemble the Hedstrom file in appearance, are less  efficient. They are longer marketed S-files. It has a double-helix cross-section and is a variation of the unifile. Textbook of Endodontics Nisha GARG 3rd ED