Interesting John, thanks! I had not considered before why there was no 180W SOX-Plus! Incidentally the DT end blackening did used to be more consistent from lamp-to-lamp. When Philips first started this process, they had a photocell beside the electrodes and this was set to cut out the cathode heating current when a certain thickness of barium mirror had been vaporised. That was a troublesome system to operate, and when it broke down it was replaced by a simple timer. Since some lamps are cleaner than others, this results in quite some variation in degree of blackening between individuals. So long as there is a certain minimum level then life will always be good. However for the short 18W, some lamps were so blackened that their initial lumen output was notably reduced.