[TI] Allegati Questions

[John Mascari]

Are Allegati documents created for EVERY matrimonio/marriage record?
Is there an INDEX within the Allegati films for each year?

I ordered an Allegati film 2017215 Item 1 from my FHC.  The film is labelled Allegati 1871-1875.
I was looking for additional information associated with marriages that I already found within the TI Database.
I could not find an index at the beginning or end of the year on the film.  I also did not find records for 1872,
1873, or 1874 on the film.  I do not see another film associated with those years.  
Does anyone know if the records for 1872-1874 are not filmed or damaged?

I also find birth and death records that do not appear to be associated with marriages contained within the Allegati records.
Why would these be recorded in two places?  
Can someone explain the primary purpose of the Allegati records
(besides being very useful in finding great genealogical information)?

Your assistance would be greatly appreciated.  Thank you.