This is an Orange Free State 1881 SG 23, the 1d on 5 Shilling Green.  It has a smaller 1, straight horizontal serif, and the obliterator bar shows the end of the row break on the right.  It is in Mint Original Gum and fine to very fine condition.  It is valued at CV L275+ in the SG Catalog.  A lovely and rare find for an Orange Free State Collection.

This item is over $100 USD so I will be shipping it with priority mail in the states and registered mail overseas.  This is to ensure that it reaches you in a safe and timely manner.