5 second fuse case seal?

[bluelake]

While searching a site here in Korea where US boat howitzers were positioned in 1871, I found a small (~0.7") round lead disk with "5 SEC" on it.  I'm guessing it was an inspection seal for a case of 5 second fuses--correct?  Attached is a pic; the metal coloration makes it hard to completely see what is written, but it is a "5" with "SEC" beneath it.
 

[Artilleryman]

Not enough detail for me to hazard a guess about what it is.

[bluelake]

This is what is on it:
 

[bluelake]

I'm wondering if it might be what's referenced here: http://www.relicman.com/artillery/AArtillery.RelicmanSalesCatalog.page6.htm (A2742) "Originally covered with a lead pull tab, this was removed before firing." 

 
 
Also:

The only difference between the lead tabs seems to be the indicated fuse time.

[bluelake]

I received a reply from an expert on the subject:
 
  That was the safety cap, which was removed and shown to the officer in charge of the battery. It was used to prevent moisture and accidental ignition. In addition there was a water cap. The Navy used numerous fuze settings: 3-1/2, 5,7,10, 15.  The 5 sec fuze was the standard fuze installed in all shells and was considered the general working fuze. The fuze would be removed and others installed for different applications.