These days, 'In Stock' frequently means the item is being held by a third party, who will fulfil the order and bill the retailer. I believe this was the case with the recent clear-out of Ampeg valve heads.
Even if a retailer has a stock item in their possession, it may be at a warehouse rather than at their retail outlet. Andertons do this - always contact them beforehand if there's something you particularly want to try, as they may have to move it from warehouse to shop.