Yes, i tested this in several lamps. The 551-nozzles are working quite fine in an 351 lantern.
It must be this way, as both lanterns are 300HK /CP-models.
Sorry Casper, but I had to edit your Link, as it was to the commercial section of Lighthouse, and commercial links are not allowed by users agreement. Now it leads to the info-page.
Best regards, Björn