First, repeating this:
Many (probably most) patent documents are available to the public. Remember that:
The European Patent Office provides a country code table to help with forming a search or understanding a patent's origin. For example:
We might expect to see any of these formats for the same granted (issued) U.S. patent, depending upon the person doing the citing or upon the database. Be sure to check on which is required --- especially if you try one that gives "0" results.
As with the record for patent US9796219, Espacenet may show other numbers besides the one you searched. In fact, the record retrieved with the search is US2017136834 (A1). US9796219 (B2) is in the "Also published field."
Also, with U.S. patents, you may see special prefixes:
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