I looked in my Honda Motorcycle Identification guide 1959-1988, published my the American Honda Motor Co.
There is no listing for a Honda with a frame number beginning with CB750G.
Because of the G in the frame number, I suspect, as Glenn suggested, that this bike was not made for the US/north american market. I have both a 77 and a 78 CB750F. When I look at the picture, there are parts that appear to be from the F model such as:
Front forks and wheel, Front brakes, rear brake and rear sets (Rear wheel too small for an F or K of the 77-78 vintage). F model engines were black for the F2.
The exhaust system is not likely from Honda.
Carbs look like 77-78 F or K models.
However, the tank, side covers, seat, grab bar, rear fender, front master cylinder, and tail light, do NOT look like or have the styling found on my CB750Fs. Some of these pieces may be K style. But, I don't have examples of those to compare to.
If it weren't for the G on the frame, I'd say it was a conglomeration bike assembled from various pieces. But, the G is significant, I think. Normal American K models won't have this, and F models would say F in that postion of the frame number.
I wonder what the engine numbers are?
Cheers,