I don't think the black strip means anything. Some have it some don't. Doesn't constitute a female or male. I have had many pullets without the strip on the head. In fact more without the strip than with. I can tell by the wings and the tail. Good luck.
In my short experience with rir chicks the longer tail and wing feathers are cockerels. They seem to grow faster than the pullets. The tails of the cockerels are pointier. After a few weeks you can see a differance in the size of the legs. I will know for sure in about a month when my test babies are grown up more. This is by no means 100% for me yet, but if they are what I think they are then it will be easy to sex. You can totally see a differance in tail feathers and wing feathers at an early age. Good luck.
THANK YOU SO MUCH FOR THE INFORMATION ABOUT SEXING RIR
CHICKENS!!!!!!!!!!! I've got 50 of the little angels that were born last Wednesday and am now going to run out to the brooder and see who's el nino and who's la nina.