and their flanges due to the respective height of the flanges their rotation is limited as is their vertical displacement flanges 305 a and 305 b each have respective top portions 309 a and 309 b which engage 450 the rimmed edge 420 as illustrated in fig4 further the flanges 305 a 305 b are vertically high enough as illustrated in fig3 d to prevent shingling and jamming thus the angle 451 formed between the respective flanges 305 b and 305 a as illustrated in fig4 is acute and limited fig4 b is an enlargement 400 b of a portion of fig4 illustrating the interaction between the flanges 305 a 305 b of neighboring fasteners fig4 a is an end view 400 a of the track of fig4 illustrating a fastener of the instant invention therein fig5 is a bottom view of a threshold 500 with the fastener 300 of the instant invention press fit therein walls 501 502 and 503 form a channel in which fastener 300 partially resides walls 502 and 503 prevent excessive rotation of the rectangular nut 321 reference numeral 570 indicates rotation of the nut as the receptacle is rotated in a counterclockwise direction as viewed.