Inside molds are needed to hold  blocks as they are attached temporarily to the mold while the garland is constructed.  Outside molds don't require blocks as the joint is shoved into the mold.  Because of these two differences, outside molds usually have mitered joints, and inside molds lapped joints.  I have only seen a couple of violins with both mitered joints and four corner blocks.  Usually they have none or two blocks when the corners are mitered.  I have never seen a violin with lapped joints and no corner blocks.

The reason the outside mold evolved and was used for a brief time was to expedite construction.  The garland could be put together and the lining put on in one application instead of several.

Two-block construction is almost always simply cosmetic and the blocks do not, as a rule, extend all the way into the corner.  As these blocks are found in the lower corners they can be seen through the f holes while the upper corners not seen.  This gives the impression of four corner block construction while in reality it is not. 

It is a documented fact that the Old Cremonese masters did not always use six interior blocks. For instance an exceptionally rare violoncello by Francesco Stradivari never had corner blocks. It is in the Great Russian State Collection. Moreover, xray and CT scans done in the recent past show that at least one well preserved Stradivarius (in a museum in Paris) does not have upper corner blocks and the neck root extends into the corpus.

An inside mould can easily be shaped to the exact outline needed for the c bout ribs to meet and join properly with the upper and lower ribs. The so called 'fake' corner block is nothing more than thin piece of spruce which upper and lower ends act as 'stabilisers' giving added gluing surfaces because of the hollow spaces created by the absence of blocks at the corners. No big deal.

In other words, the presence or absence of anything inside any Cremonese, Brescian, Mantuan, or Venetian masterpiece (or any other VSO )has absolutely nothing to do with the authenticity or 'quality' of same. Bartolomeo Giuseppe Guarnerius would have been better off to not have used anything inside some of his instruments, as cobbled up as some interiors appear to be.  

It is the skill involved in the desgn/layout of the patterns and the arching/thicknessing of the plates that counts. (Not to mention the beautiful varnishes used by the old masters.)

Cheers!