Answer:

The "toaster tank," is a BMW motorcycle gas tank with chrome side panels introduced only in 1972.

The chrome paneled 4-1/2 gallon tank replaced the tall black 6-1/4 gallon tank. Chrome side panels or battery covers, with stripes painted black or blue came with the "toaster tank" machines. Some machines also had shock absorbers with aluminum top covers. The nickname "toaster" is in reference to the gas tank's resemblance to the toaster, a kitchen appliance. BMW: A Celebration, attributes the styling change to the appointment of Bob Lutz, the American automobile executive and motorcycle enthusiast, as BMW sales director.

The blank tank was standard in the USA only in 1970-71,

1972 had only the toaster tank, though some dealers replaced the chrome panels with rubber pads.

1973 had a choice of size, but none were chrome like the toaster tank.

Toaster History:

September 1972

The toaster! Chrome 18 liter tank, chrome side panels, engine type on engine case in aluminium on black, straight exhaust damper, extra colors: red, blue, green and curry. Also a new saddle and new handrails.
R50/5 2903624
R60/5 2938705
R75/5 2982741

October 1971
Wider rearwheel rim, 2.15B-18 instead of 1.85
R50/5 2903756
R60/5 2939207
R75/5 2983280

Februari 1972
Changed crankshaft and bearing shells (stiffer crank).
R50/5 2904190
R60/5 2940740
R75/5 2985208

Spring 1972
Changed rocker shaft support brackets and hardened steel shims, because of the noise
R50/5 2904721
R60/5 2942632
R75/5 2989472

Summer 1972
Vaccuum ports on the CV carbs
R75/5 2992320

21 August 1972
End of the toaster!
R50/5 2905616
R60/5 2945288
R75/5 2994267