The primary difference will be that the 1971 covers the B20E engine while the 1973 will cover the B20F which is the lower compression version of the B20E. So, some of the engine tuning specs will be different and the emission controls, primarily the Evap will be slightly different. If you can find the tuning specs for the B20F engine with D jet the 1971 service manual should be just about as good as the 1973 service manual.

Volvo in North America used to sell new editions of the factory service manuals for the vintage Volvos. I ordered a brand new paper green book for my 1971 142E about 10 years ago from their web site. Every time I search I get directed to the Canadian site - which just wants to sell me a new car. You might have better luck or maybe Volvo does not offer that support anymore.