There's a simple explanation: PLOT 97 (draw rectangle) was not implemented on the original BBC Micro Model B - that used PLOT codes only up to 87. I'm not absolutely certain when PLOT 96-103 were introduced, it may have been the Model B+ or the BBC Master.
On a Model B you must draw the rectangle as two triangles (here in the centre of the screen):
10 MODE 5
20 MOVE 592,462
30 PLOT 0,0,50
40 PLOT 81,50,-50
50 PLOT 81,0,50
Richard.