Telect's 3-module ELF chassis (P/N ELF-0000-2400) uses universal mounting brackets supporting both 19" and 23" wide mounting.  If you require a replacement set of mounting ears, you will need to order the left and right ears separately.  P/N 123890-1RC is for the left side bracket when facing the front of the chassis and P/N 123891-1 is for the right side bracket.