There are a couple of misconceptions regarding heavy duty dana 44's and later model high pinion units.
The hub and lockout which use the large external style hub was found on early "highboy" f-two fittys only. This was available on drum equipped trucks up until 75. 76 and 77 trucks also used the external design and was most common in 75 and 76 with a few early 77's in the bunch. One could find a randon dana 60 in some crew cabs and a rare standard cab or two, but these low pinion front axles bring little to no desirability, and are often mistaken for the more desirable 78/9 high pinion.
After the split in 77, all f-two fittys used a high pinion front axle and only had a small hub and lockout. These are not considered to be a HD front axle, and truth be known, there was nothing HD about any dana 44, but only units found in highboys were recognized as or as being identified as "HD". Again this is for the F-250 only.
Alright so in 78 and 79 the F250 was available with two front axles, one the dana 44 high pinion and the second was the high pinion dana 60. This high pinion dana 60 was only optional with the snow fighter package and had to be ordered this way.
Not until 79 was the F350 4x4 ever available, and this came standard with the front dana 60.

If you have a 78 truck with a large external hub and disc brakes, chances are very high that this was swapped out for an early axle some years back.
Sounds as though you have a 76 or 77 front axle under your rig.
Either the axle is different, or someone has changed the VIN, and you are looking at a 78 door / registration on a truck that is a little older than you think.
What is considered to be a "highboy", will have a divorced transfer case, and low pinion front axle. Later trucks with a high pinion front axle will have a married or direct mount transfer case. None of these 78/9 trucks would have had early style external lockouts.

Any pics of yours?


Example of early style external hub: