Thanks for the catalog views. No doubt the older RB brass nozzles were the 18 tpi coarse-thread types, given the A-7 adapter shown. I suppose 18 tpi wasn't a good choice for plastic nozzles, so that would never had been considered a starting point for plastic popup nozzles when the 1800 series was conceived. I guess the 28/27 tpi difference was as likely coincidence as anything else. I don't imagine anyone at the time thinking in terms of an 1800 brass nozzle not fitting on industry-standard all-brass popup bodies.