What is the difference ?

 

Time for Some Homework! http://omega-constellation-collecto...n-earth-would-you-buy_115322299861451780.html

 

These two articles, also from Desmond and linked from the website above, give a more direct answer to the question:

http://users.tpg.com.au/mondodec/movement2.pdf
http://users.tpg.com.au/mondodec/movement3.pdf

 

Funny I was lookin at that page before I checked back with you guys

"The 505 is basically the same movement as a 501 but with added jewels"

 

Thank You Sir !

 

In answer to your post title - 46.

The 505 is a 24 jewel chronometer rated movement based on the caliber 490 family.
The 515 is a 24 jewel chronometer rated movement based on the caliber 550 family.

Essentially, the 551 is a later refined version of the 505.

505 (with SC) and 551 data for reference.


Thanks to Dr Ranfft

 

is the sub second a small second hand dial ?

 

Well you go to http://www.ranfft.de/cgi-bin/bidfun-db.cgi?10&ranfft&&2uswk and find the movements and tell me about sub-seconds, sweep seconds and centre seconds (SC) layouts.