A retrofit seat back assembly (150, 250, 350, 450) for a stowable seat having a seat back frame (114, 314, 414), the retrofit seat back assembly (150, 250, 350, 450) comprising: a supplemental recline panel (152, 252, 352, 452) having an upper end and a lower end; a pivot member (275, 375, 475) disposed proximate the upper end and configured to pivotally engage the seat back frame (114, 314, 414) of the stowable seat; an actuator (254, 354, 454) disposed proximate the lower end and configured to operably engage the seat back frame (114, 314, 414) and move the supplemental recline panel (152, 252, 352, 452) between a retracted position and a deployed position; and a bias element (260, 360, 460) configured for disposition between the supplemental recline panel (152, 252, 352, 452) and the seat back frame (114, 314, 414) and configured to urge the supplemental recline panel (152, 252, 352, 452) toward the seat back frame (114, 314, 414), wherein the bias element (260, 360, 460) comprises a spring (263, 363, 463) having a first spring end (266, 466) connected to the supplemental recline panel (152, 252, 352, 452) and a second spring end (267, 467) configured for connecting to the seat back frame (114, 314, 414); characterised in that: the bias element (260, 360, 460) further comprises a main body (261, 361, 461) configured for disposition within a lower aperture (340, 440) of the seat back frame (114, 314, 414), the second spring end (267, 467) is connected to the main body (261, 361, 461), and the main body (261, 361, 461) comprises a flange configured to engage a first surface of the seat back frame (114, 314, 414) and is configured to receive a retaining clip (264, 464) proximate a second surface of the seat back frame (114, 314, 414). a supplemental recline panel (152, 252, 352, 452) having an upper end and a lower end; a pivot member (275, 375, 475) disposed proximate the upper end and configured to pivotally engage the seat back frame (114, 314, 414) of the stowable seat; an actuator (254, 354, 454) disposed proximate the lower end and configured to operably engage the seat back frame (114, 314, 414) and move the supplemental recline panel (152, 252, 352, 452) between a retracted position and a deployed position; and a bias element (260, 360, 460) configured for disposition between the supplemental recline panel (152, 252, 352, 452) and the seat back frame (114, 314, 414) and configured to urge the supplemental recline panel (152, 252, 352, 452) toward the seat back frame (114, 314, 414), wherein the bias element (260, 360, 460) comprises a spring (263, 363, 463) having a first spring end (266, 466) connected to the supplemental recline panel (152, 252, 352, 452) and a second spring end (267, 467) configured for connecting to the seat back frame (114, 314, 414); characterised in that: the bias element (260, 360, 460) further comprises a main body (261, 361, 461) configured for disposition within a lower aperture (340, 440) of the seat back frame (114, 314, 414), the second spring end (267, 467) is connected to the main body (261, 361, 461), and the main body (261, 361, 461) comprises a flange configured to engage a first surface of the seat back frame (114, 314, 414) and is configured to receive a retaining clip (264, 464) proximate a second surface of the seat back frame (114, 314, 414).