These are Vixen type dovetails. They are designed to hold light equipment such as DSLR cameras. I use it to place a DSLR on my equatorial mount to take wide field astrophotos.
There are three different versions:
- 200mm dovetail with a 150mm groove (M6 - 1/4") for general purposes,
- 120mm dovetail with a central drill to place a 1/4" screw to fix the DSLR.
- 200mm dovetail with a central drill to place a 1/4" screw to fix the DSLR.