Hi, I work for a sailmaker who has a singer 457 A105M which is a great little workhorse.   I am looking to purchase something similar as I am having continuos  timing problems with my Pfaff 138.   I have found singer 457s with different sub headings of A, G or U, such as singer 456 A 505G-L.    
My problem is that I can't find an explanation for these headings.   
Can anyone help? 
Many thanks!