The article "Final Metro Delivery" in Airways magazine Issue 64 states that Metro deliveries before the Metro II was introduced totalled 18. The Metro production list shows that by the end of 1974 22 Merlins had been built. However a photo on Airliners.net (here) of the 33rd Merlin IVA built (Fairchild c/n AT-038, built in 1975), shows it to have the earlier round windows.