Thank you all for your replies!

Quote Originally Posted by Louis Pacilla View Post
Check this out
http://books.google.com/books?id=1mc...lenses&f=false

I don't know if this helps but in 1864 the 3A was 12" FL . I don't know if this is measured from the back or stop. But at least it shows how FL/model # changed with over time with most manufacturers including Voigtlander & Dallmeyer.
The dimensions of the lens elements of Ross No.3A in this book are the same as of my Ross lens.
But the difference of the focal length is still strange. I'll try to measure the focal length of my lens using more scientific methods an then we'll see the truth.

But there is another question arised: how to measure the maximum aperture of such lens with different lens elements? As usual, dividing the focal distance on the measured diameter of the front pupil?