Neither Tice or Albert list this variety of button. Tice has very little that might apply here. PBK's example is very close, however the ring is centered while your's is to the right. (As well, your rope falls over the anchor on the first turn). Albert mentions variations to both the device and backmark on PBK's example (Albert 131A or B) . The example that looks closest to your's however, is Albert 132 which is pictured here. The only variation is how the rope passed over the anchor (and the lack of any backmark). It is possible maybe that variations of this design exist also which are unlisted in Tice or Albert?