Alcan Primer #217B ???

Yep. It's a Berdan primer, so it has no anvil so you have to decap Berdan-primed brass for it. The name 217 comes from the fact 5.5mm is 0.217 inches. That makes it wider than a Boxer large primer, which is nominally 0.210 inches.

From Dave Cushman's site:

"European-made .303 British, and South African, is often made with .217" primers (5,5 mm) and the PMC and RWS numbers of this size are suitable.

China has made 7,62 x 54R with a .217" primer for some forty years now."​
 
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