[HECnet] "illegal record size" in RSX?

I've had my head deep in the RMS internals and I should have recalled this. The XAB (eXtrended Attributes Block) defines: literal XAB$K_RT11 = 1; literal XAB$K_RSTS = 2; literal XAB$K_RSX11S = 3; literal XAB$K_RSX11M = 4; literal XAB$K_RSX11D = 5; literal XAB$K_IAS = 6; literal XAB$K_VAXVMS = 7; literal XAB$K_TOPS20 = 8; literal XAB$K_TOPS10 = 9; literal XAB$K_RTS8 = 10; literal XAB$K_OS8 = 11; literal XAB$K_RSX11MP = 12; literal XAB$K_COPOS11 = 13; literal XAB$K_P_OS = 14; literal XAB$K_VAXELN = 15; literal XAB$K_CPM = 16; literal XAB$K_MS_DOS = 17; literal XAB$K_ULTRIX_32 = 18; literal XAB$K_ULTRIX_11 = 19; literal XAB$K_RMS11 = 1; literal XAB$K_RMS20 = 2; literal XAB$K_RMS32 = 3; literal XAB$K_FCS11 = 4; literal XAB$K_RT11FS = 5; literal XAB$K_NO_FS = 6; literal XAB$K_TOPS20FS = 7; literal XAB$K_TOPS10FS = 8; literal XAB$K_OS8FS = 9; literal XAB$K_RMS32S = 10; literal XAB$K_CPMFS = 11; literal XAB$K_MS_DOSFS = 12; literal XAB$K_ULTRIX32_FS = 13; literal XAB$K_ULTRIX11_FS = 14;