I saw a link posted earlier today and saw a black glass 'Eye of the Tiger' and thought gee that looks an awful lot like Sinbad.  And sure enough its the 2 player version.

My question is why did they bother making a 2 player and a 4 player version of a machine .. I can understand upgrading titles from EM to SS which is a bit of a stretch ... as they should have just moved on to a new title.  But what I don't understand is why they would have gone to all the trouble changing the art work and associated items just to add two players.

ref.
Eye of the tiger
http://www.ipdb.org/machine.cgi?id=802
Sinbad (EM)
http://www.ipdb.org/machine.cgi?id=4673
Sinbad (SS)
http://www.ipdb.org/machine.cgi?id=2159

Movie tie in .. with ray harryhausen stop motion (always a plus)
http://www.imdb.com/title/tt0076716/

Wonder if there is a tie in or easter egg in TOTAN for this?

$$$$$$$$$$  

money saving ... they release the 4 players first so if you have to have the latest and greatest, it's more $$ .. then they release the 2 player for cheaper, later

Yep, as stated by MrHide. Market / $$$.

Machines were designed/built with local market demand in mind,
Europe had different requirements than North America apparently,
with regards to 1,2, or 4-player games.

Another variant also seen in older EMs are the 'replay' and 'add-a-ball'
different versions (such as 1968 Williams Daffie / Doozie for instance).
This was done to accomodate the various state laws with regards to 'gambling'.

In the 1970s, manufacturers figured-out a way to make this functionality
as an operator-adjustable feature - as the match in or out, for instance,
in one machine. Of course they still needed different models for 2 or 4 players.

One interesting 'modern' example of this is the 1982 Bally 'Speak Easy' (2-player),
remade later as a 4-player for the European market I think...

Then again, 6-million $ man has 6-players...
As well as the older EM Six-Stick I think.
Not many early 6-player pinball machines!

Cheers,
- Sylvain