It is not ok to use that type of descending ring for fixed anchors.
SMC Descending Rings are a one-piece aluminum ring which are intended to be placed at the top of a pull down rappel in place of a carabiner in order to facilitate recovery of ropes.
SMC issues the following for care, maintenance and retirement schedule needs of their descender rings:
CARE, MAINTENANCE and RETIREMENT SCHEDULE NEEDS
Always inspect Descending Rings before each use and periodically while
in storage. The user, depending upon their specific environment and
storage methods, must determine the period between inspections.
Inspect for cracks, warping, deep gouges and worn areas, making sure
that what may appear to be a scratch is not actually a crack. Look for
sharp edges or rough areas that might abrade a rope. After each use,
remove all dirt and allow Descending Rings to dry in a warm place
before storing. SMC Descending Rings will continue to provide
reliable performance only when used safely and properly maintained and
stored. It is also suggested that the user maintain a permanent
record listing the date and results of every usage inspection.
We recommend the regular inspection of all rescue equipment and
strongly suggest retiring gear when ANY of the following applies:
- Regular inspection reveals warping, cracks, deep gouges or any wear.
- It is physically damaged or no longer functions as when new.
- It has been subjected to an abnormally high loads, such as in a fall or exposed to heat sufficient to alter its surface appearance.
- You are not completely satisfied that it meets the needs of its intended use.
- The history of the gear is unknown or otherwise in question.
Source: [PDF] SMC Descending Rings #81600, Seattle Manufacturing
The history of any SMC descending rings that you may find attached to an anchor is unknown and therefore it is strongly suggested by the manufacturer that you retire those rings.
Do your fellow abseilers a favour and remove those rings from any anchors you may find them attached to. Either destroy them, or mark them as retired and use them to hang your hammock.
Lightweight aluminium descender rings are designed for quick descents of alpine routes, they are not intended to be used for heavy duty applications. The improper use of any piece of equipment is unsafe:
Use the right tool for the right job! Thin alloy rap rings are not intended for heavy-duty use, carefully inspect descender rings and retire if they show any sign of wear.