QOB circuit breaker have been manufactured since the 1950's. Sometime around 1986, the QOB circuit breakers were given a new bolt-on connector. This design change did not affect their use in NQOB panels. However, the NQOD and NQ Panelboards will not accept the older design connector that was on the pre-1986 QOB breakers. There is no problem using the new bolt-on connector QOB breakers in NQOD and NQ Panelboards. See the below pictures showing the pre-1986 connector and the post-1986 connector.