Aberto, your unit is ‘standard’ ZF 10544.17.043.01 LSD and I think that I finally understand that ‘small unit’-‘big unit’ is question of diff housing, so yours must be before 1972 series 25 (15?) splines one. (First series of 10551 had ‘small’ diff housing if I remember well). It is perfectly capable of governing GTAm power under ‘normal’ conditions, but you can improve it for major torques as well. To discover its ratio, you should open it to see number and thickness of plates. Standard 2-4 plates (1,90) gave 25% closure (optimal value for road ride) that you can increase adding one more plate (2.0-2.10) to major it up to 40% (already racing condition) and even (one plate more) up to 60-70% for extreme track use but not for street driving. Be careful in using diff oil. It MUST be 75W-90 (red line) and NOT 75W-140 (though indicated by producers) as it could damage seriously synchro. I would suggest to change the set of plates with new ones to obtain 25% revised unit, or by new one (attention! serious prices) at Alfaholics or Classic Alfa. Good ride!
For Lastforry; the numbers on LSD housing are ZF production code for ITALIAN MARKET, different from German standard codes. Send letter to ZF, and they will give you the right type of codes for spare parts.
P.S. For diff works you need a special tool set!