If you've come across a used Boss Frog rollbar and installed it in your Miata, the seatbelts sometimes wont retract as well as they did due to binding. These bolt-on adapters help the seat belts slide a bit easier. Installation does require cutting off the OEM belt guides.
These are for use on 2005 and earlier Boss Frog and Hard Dog Rollbars. BF started including them after 2005 until BF closed their doors a while back.
Price is for the pair (Single piece also available).
Assuming that you tilted the seatbelt reel toward the inside of the car and trimmed the plastic bits properly per the Boss Frog instructions, these seatbelt adapters will help improve the belts retraction. Factory level of retraction is a thing of the past though... you've modified your car beyond that ...
Original Boss Frog instructions are available for download here: BOSS FROG INSTRUCTIONS
Miata Seat Belt Adapters