Here is the deal. Not one but two TPA services rejected this signature as authentic. I was and still am convinced this is as authentic as it can get. Sometimes this hobby can be very frustrating!
The archive shows a mix of Bowies - real and several other LP's in silver paint pen and blue paint in one hand - not Bowie's. Have you checked other stock?
When I look at a seller's autographs I only review the ones that interest me. I have purchased more than this item without any problems. Honestly, I am only showing the source as part of provenance. I also have received a couple of private opinions on this Shaw that were positive.
I believe this Shaw is authentic. If a fake, there was a lot of unnecessary effort to "sell" the signature. But, I am open minded and would like to see the reasons why this signature is suspect.
We all talk about how TPA cert all these bad autographs. What about when they reject likely genuine ones?
I try to check everything I know to look at. I have seen the hand that made those silver Bowie's before, on eBay, and have many scans of them. I'd be wary in general - there are a number of them over time.