These National tubes were manufactured in England in the 1970's and are exceedingly scarce to find. From what we can tell, only Burroughs and National manufactured these tubes and no other known substitutions for the this model of nixie. As far as we know, there are no aftermarket readouts for these older Drake lab quality receivers. These nixies will be important to have available to either replace or to have on hand in the event you want a backup set in the future to replace tired display nixies.