If you have a serial number beginning with 18, 19 or 20 you will use one version of these parts.
If you have no serial number or any other serial number besides the ones above you will use another version of these parts.
The location of the serial number is found on the underbelly of the center housing.
Fits all tubular sights with the exception of the HD Micro. The elevation knob for the Matchdot II and the Ultradot 6 are different-See Elevation Indicator in parts. Trim ring will not fit the Ultradot 6.