If it is a H145 unit, then it is in all likelihood from a B10 or B20 vehicle.

These used a fixed spacer sleeve with shims to control the preload on the pinion bearings.
This method was costly & slow to produce but the pinion seal & the pinion flange can be changed in-car.

The following iron H145 diff used the collapsible sleeve method of construction to control the pinion bearing preload & this proved much cheaper & quicker to produce, however the sleeve is a single use item & anytime that the pinion nut is loosened for any reason, this sleeve must be replaced.

Unfortunately this involves removing the diff from the axle housing, then removing the ring gear with its carrier, then removing the pinion in order to access the sleeve for replacement.

The whole diff must then be re-assembled with the relevant tools to ensure a correct setup before refitting to the car.

What a pain.

Oh yeah, the B10/20 H145 diff was manufactured with 4.11 to 1, 4.375 to 1 & 4.625 to 1 ratios.
It is my understanding that the entire internal components from the iron H145 can be fitted in order to gain the 3.9 to 1 ratio if necessary. [or desired]