Correct - 1973 would be the clear plastic cover with small block lettering as shown in your picture. This is the standard cover of that era and used on both American and English bicycles. There is an earlier clear cover with large logo/lettering from the 1960s, and then there is a later cover made of black plastic material (later 1970s and into the 1980s). The later era ones with the black cover often have the black, plastic-covered shifter lever. The clear plastic ones should have the plain metal shifter lever.
The black plastic cover is commonly seen as a replacement part because the clear ones broke and fell off, needing replacement. The clear ones have not been reproduced, to my knowledge. They usually are cracked to some degree when you find them. Best bet for an original is eBay or the used parts bin from a bike shop that has been around a long time. Another option is to replace with an earlier, all-metal Sturmey shifter.