The rail portions of {TAIL ~ the frame} each have an upright outer wall, an integral downwardly slanted inner wall and {HEAD ~ a hidden outwardly open slot} therein between the outer and inner walls 
The rail portions of { TAIL ~ the frame } each have an upright outer wall , an integral downwardly slanted inner wall and { HEAD ~ a hidden outwardly open slot } therein between the outer and inner walls
OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH