In the left image, which he labelled "Dundee May 1919", HJH is wearing standard uniform worn by ratings in the RNAS. By this time the RNAS had ceased to exist, having been incorporated into the RAF in April 1918. It seems that old uniforms continued to be worn, perhaps until they were worn out, or as apparently in this case, through a sense of pride.
Below, the three men (HJH on left) are wearing the new RAF uniform, and also in the bottom image, where it seems to being used to good effect !
The image of the three men is captioned "HJH & Co. taken at West Ferry. 'D' Dorm."