It appears to be late K2 or early K3. Frame number will tell for sure: K3 was made starting in 6/72. K2 was started in 6/71.
Now that you've stirred that pot...
The OP may not be aware that at that time, different models of the CB750 could be legitimately titled 1972, but yet be actually different models. AS you say the frame number tells all as far as the model is concerned, by the titles date can be ambiguous.
While we're at it, the K2 got the bigger tail light over the K1. As on the bike in the picture.
He asks about the tank stripe, and I see my Honda ID guide does list it as being Gold. But I can't imagine a gold stripe on the sunrise orange tank.