Skirting part of R2-D2 kit on the Nostromo

leading from 

twenty 16th scale r2-d2 kits

a) The skirting of R2D2 at the bottom of the main body

piece from the kit for R2D2's skirting

b) Where it is found on the Nostromo

Tha same part of the R2-D2 kit with its sides clipped

One of Jon Sorenson's transparencies of the Narcissus. The piece
 is to be found in the top right area seen of the Nostromo here