bar 421 and locking elements 423 425 427 toward the housing end having the lever 439 i e to the locking position in this embodiment the closing arm 549 alone pivots the hinge plates 417 419 for closing the ring members 449 the locking elements 423 425 427 do not cam the plates 417 419 to pivot unlike their counterparts in the first and second embodiments fig1 17b show a fourth embodiment of the present invention the mechanism of this embodiment is generally described by reference numeral 601 parts of this embodiment corresponding to parts of the first embodiment are indicated by the same reference numerals plus u201c 600 u201d parts corresponding to parts of the third embodiment not included in the first embodiment are indicated by the same reference numerals plus u201c 400 u201d this embodiment is substantially similar to the third embodiment but in this embodiment a first end 707 of a tension spring attaches to a travel bar 621 while a second end 665 attaches to a detent 735 in a raised plateau 631 of a housing also in this embodiment the travel bar 621 is shaped as a rigid channel having a flat web and two side flanges it includes three locking elements 747 that each include two locking flanges 749 integrally attached to side flanges of the travel bar the locking flanges 749 project downward.