Using a model for the bundle F 2 M of semi-holonomic second order frames of a manifold M as an extension of the bundle F 2 M of holonomic second order frames of M , we introduce in F 2 M a principal bundle structure over F 2 M , the structure group being the additive group A 2 (n) of skew-symmetric bilinear maps from R 2 × R n into R n . The composition of the projection of that structure with the existing projection of the bundle F 2 M of non-holonomic second order frames of M over F 2 M provides a principal bundle structure in F 2 M over F 2 M . These results close an existing gap in the theory of second order frame bundles.