Another question I have... I bought a Savage XL 5.9 GTXL-6 - DIFFERENTIAL 29t Flux GT-6 Front or Rear Diff HPI 160102 the guy I bought it from said it should work and it did fit! But that couldn't be the reason it's going back wards right??
HPI#160102 is the number for one of the many 1/8 Savage Nitro/Flux Monster Truck models that HPI offered over the years.
I am inclined to bet that the differential you have in your hands was taken out of a HPI#160102 Savage Nitro Truck. You see, in certain circumstances, RC car manufacturers, just like their slightly bigger brethren (think Plymouth, Saturn, Oldsmobile, Pontiac...) lose the game of super capitalism and inevitably go under. In the RC context, in such situations, after all pensions have been rescinded and all patents and trademarks sold for a few bucks, there still remain thousands of RTR units as well as car kits in neat boxes in the warehouses and inside stores... That's when a type of business folk come in and buy those remainder stock at a very good price so that the newly bankrupt enterprise can more easily breach its contract with the warehouse owners and terminate the contracts it has with its own dealers and distributors worldwide.
Anyhow, from this point on, RTR RC cars and RC car kits lose those designations and turn into "core parts" and "dead stock". In the former category fall differentials, wheels, TVPs, center gearboxes, dogbones, drive cup shafts, nitro engines, filters, engine stands, spur gears, clutch bells, ESCs, motors, batteries, shock absorbers and a few other things. In the latter, fall the little Phillips screws, the washers, body clips, those worthless ball-bearings and every other tiny part that doesn't cost more than a buck at retail.
And so, it becomes easier for us after a while to even remember that a company called HPI even existed. At the same time, a few large warehouses have become empty before the end of their lease contracts, some folks have earned a bit of money and you got yourself a bitching differential which if you shim well and maintain properly, should not fail you anytime soon...