Looking up the unifilter catalog R31 = TT267-155S a Z31 = TTN-4200S if I lookup a cross reference for the K&N for the 280z (USDM) I find the following.
K&N = 33-2010 (correct for the 280z and Z31 300ZX) this cross references to Ryco A345 which cross references to TTN-4200S
Going backwards from the R31 I get
TT267-155S = Ryco A360 = K&N 33-2031 which as K&N have the sizing on their site is thinner and other dimensions are slightly different
K&N 33-2010 = 35mm H, 270mm L, 156mm W
K&N 33-2031 = 29mm H, 279mm L, 167mm W