Genuine Birkenstock thong stems used, most recently, in the Gizeh and Ramses styles. The thong stems come in black, white, and tan. Sizes are small and large. The small thong piece fits Birkenstock sizes 35 to 40 and the large fits Birkenstock sizes 41 to 48.
Unsure about your size? Check out the Birkenstock Fitting Guide to help you. https://www.birkenstock.com/us/us-service-fittingguide.html