at some point (I believe the L model) the spinner became required equipment, making the skull cap no longer an option. Be sure to check your aircraft s/n to see if the skull cap is even an option.
That is correct. The Skull Cap spinner is not allowed on the "L" and "M" models due to the change in the design of the cowling. I have seen a photo of an "L" with a skull cap over on that other site, but it's illegal.
The Parts Manual states the Skull Cal is optional on all Cessna 150s up to S/N 15072003. That's the last "K" model.
I'm on my 3rd spinner, the one that came on the airplane had a hole in the nose. Another club member gave me his because he was doing the 150 hp upgrade. That one lasted a couple of years and it cracked. I found the one I have on eBay and got it quick for a good price. The aft bulkhead also cracked and I spent the bucks and bought a new one.
The L/M spinner is the hardest one to find. If you get desperate ..... Cessnaparts.com has them $1,465.71
(mention you're in the Cessna 150-152 Club for a discount)