For searching by Scott number (or other catalogue numbers) just use the number and letter. Don't use Scott or other terms. Using the # (hatch) sign is ignored by
ebay's search.
The best search for 1590A would be 1590* (asterick) as that is a wild card and shows all titles having 1590 as the start of a term. Then you can narrow down the results (if too many) with going to specific categories or adding extra terms or minus terms like '1590* (us,usa) -greece' .
When you use 'Scott 1590A' it means show me the titles just having Both Scott AND 1590A. It does not mean show me any titles with Scott OR 1590A.
In order to do that you would have to use a word/term plus the terms you want in parentheses separated by commas.
Example:
stamp (scott,1590A)
would return all titles with the word Stamp AND either Scott/scott OR 1590A/1590a.
Just putting the two terms together in parentheses does not work unfortunately.
There is another way also. You click on Advanced Search next to the search button and choose from the drop down menu 'any words, any order'. Extra steps involved so most folks don't want to do that, including me. Short and sweet is best in searching.