I was told that model D781SB is in fact hub D771SB. Is that true?
Yes, that’s true. Please check below the history / evolution of indicated hub model:
It’s obvious that it all started with model D711SB which was a single-axle hub, meant for standard QR use only (non-convertible hub).
But its upgraded version XD711SB, later renamed to D771SB, was already convertible hub with multi-axle system.
Model D781SB was introduced just recently, only to clearly define side cap types: threaded vs press-in type. D781SB was meant for press-in side caps while D771SB was for threaded side caps.
All this is just history now because in 2017 hub D771SB (and rear hub D772SB) were discontinued and replaced by model D791SB (D792SB rear).
New hub D791SB is in fact a hybrid / mixture of hub D771SB (same flanges) and hub XD611SB (same hub shape).
Speaking of XD611SB, this hub uses exactly the same side caps like D771SB so due to threaded vs press-in type of side caps this model has a sibling hub called D551SB (for threaded side caps) while XD611SB comes with press-in side caps.
There is the same analogy for rear hubs: new 2017 model of D792SB is a mix of discontinued D772SB and XD612SB. For MTB segment, where stronger hub is required, there is a new convertible hub (year 2017) called D462SB replacing D772SB and D882SB (also old hub D992SB).