How close to any type of foundation were these found? Are there any signs of old lead pipe plumbing or cast iron drain pipe? Look up plumbing techniques for the 40s & 50s. Cast iron pipes were joined with oakum (if memory serves correctly) then a band type form was placed around the joint and lead was poured in sealing the joint. Lots of times lead splashed out or ran out over the top of the form.
If the soil is acidic like found in Arizona or Nevada, lead can oxidize like this in 20 or 30 years. As stated above chemical tests are expensive. You might try talking to an old plumber. If you have a local union, they can probably put you in touch with some of the "old boys". For the cost of a coffee or a lunch you may come up with a wealth of info.
Remember, one of the very best research sources are the old timers. They are a wealth of info, because they were there and lived it. An hour listening to the old timers will more than likely give you more knowledge than hours and hours in the library.