Yeah, the arrma one uses a steel end cap. Fitment can be a bit problematic though...Some have said one type fits better within the diff bulkheads. The other fits better in the center. IDK what's actually true in regard. I only use the Arrma metal variant for my center diff. I keep the stock plastic for my Fr and Rr diffs. right now.
>>>I will say that HR had made the original one out of alloy completely at one end, this caused one output cup to wear it out excessively fast. NG.
There was a newer HR one with a Steel Insert in place on the Alloy end now, that doesn't wear out. Perhaps you are seeing an old and newer released one.
Make sure to get the one with the Steel Insert for obvious reasons cited above. If that distinction is clear enough as described by the seller and according to its part number. Go to HR's web site to confirm the correct revised version.
Hopefully others will chime in.
I've used both, I like the one piece better because that o-ring on the two piece tends to leak. Also, you must put a shim behind the sun gear because the pin will eat into the case.