The 510 is a two-line telephone. T&R 1 is on Green/Red, and T&R 2 is on Yellow/Black.
Are you familiar with
www.Paul-f.com ? Paul Fassbender is a friend and fellow member of TCI and ATCA. His web site shows thumbnails of Western telephones, a brief description, and generally, a BSP reference. Without his site as a resource, you are at a disadvantage. See, for example,
http://www.paul-f.com/we500typ.htm There are two kinds of plastic used in WE phones of that vintage: "soft plastic" and ABS. Soft plastic phones came first. They are very collectable, and bring bigger bucks from collectors than the later ABS phones. They are distinguished, as you have discovered, by the "thud" sound when tapped, and by what is described as a vomit odor when the caps are unscrewed from the handset.
From a distance, assuming a stock telephone, you can immediately identify a soft plastic phone by observing that the transmitter and receiver caps do not have a center hole in the perforation pattern. "Hard plastic" phones have the center hole punched out.
Hope this helps.
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