By the way, an interesting detail, the Z-21 follows the Z-20-naval variants numbering system, namely no. 62TP with T standing for type or variant and P for individual prototype.
Z-20J as the first major variant with 62
1X: at least 4 prototypes are known (
,
,
,
)
Z-20F as the second major variant with 62
2X: also at least 4 prototypes (
,
,
,
)
Z-21 as the third major variant with 62
3X: 6231 (unconfirmed),
,
However since the original Army Z-20 use a simpler system based on 63P (with so far at least likely 5 prototypes built (631 unconfirmed),
,
,
,
) it could indeed be that the Z-21 is more a naval project.