That does look like a factory LSD and if it is they are a very nice unit. I haven't worked out a way to distinguish between a few of the more common gear type LSDs, they all look pretty similar, and often people have no idea what they have in the first place. Sometimes they foul around the crown wheel bolts so you need to check the fit in both cases.

I honestly reckon the 4.9 is too short for Collingrove - traction from the steep start line will be difficult and you'll forever be changing between 2nd and 3rd with the extra power. I'd be chasing a hydro 4.4:1 personally. Mind you I know a fella with a turbo DC2 running 4.9 and is very quick around Mallala. There's always compromises - the setup requirements for collingrove are much different to mallala. Do you intend to get serious about a specific class of competition? If so, choose one or the other to build your setup around.

The cable .742 5th set doesn't work with the hydro main and countershafts. I have .714 hydro LS 5th ratio sets in stock if you want a more comfortable cruise, but not necessary unless you use the 4.9 or 4.785 FD.