Boa ConcealedUK made patented restrictor designed to work with any standard track friction stay. Can be used as either a removable or permanent restrictor device. Fully concealed in the friction stay track. Locking blocks are made of die cast zinc, all other parts are stainless steel. Available in domestic or commercial versions. No additional fixings are required. No drilling is required. No need to stock restricted stays. Can be retrofitted on site.