Here is an unusual registered cover from Ipswich to West Chester Pennsylvania- forwarded to Philadelphia.
The sender is Whitfield King & Company stamp dealers under War Office Permit No. C1 and arrived at West Chester April, 1918 The cover received a Cat No. 3427 Form B label to alert the Postmaster that the package was supposed liable for customs duties. The label was applied only to suspected dutiable mail that was destined to a US address beyond the point of entry where there would be a customs officer. The Ipswich and Philadelphia cancellations are indistinct and/or incomplete but there are readable West Chester cancels of April, 1918.
Additionally, the British Censor tape for number 6524 was tied to the envelope by a boxed P.C. wax seal .
So on this cover we have a GB War Department Permit, a British censor label tied with censor's wax seal and US Customs Form 3427 B