The dipsticks are reversed; the short handle goes with the SH box and the tall handle goes with the long tail case box.
SH means single helical, so if it is on a double helical box it is someone’s modification. Look for a JL or SL number in the left rear corner of the main case.
The lack of a rear mount means it would not be suitable for SS or Mark IV or V or XK120, but would work in a Mark VII or XK140.
But the speedometer drive is at the wrong angle for Mark VII and XK140, and we normally do not see the name Standard on Mark VII boxes, so it may be out of some other make such as Standard or some other British car that also used Moss boxes. But it is interesting as now we know the reason for the notch in the top cover, to clear the speedo cable if it was located there.
The long tail case box is suitable for SS saloons including my '38, SS100, Mark IV, and would fit in Mark V and XK120 although a double helical is not desirable in those models.
The number cast into the side may be 40304 which is the part number listed for '38 SS saloon main case. For SS100 most parts are the same, except for the rear speedo drive cover and gears, so it may have had different ratios. If you are selling them, you might want to determine the gear ratios, by putting it in 1st, 2nd and 3rd and turning the rear flange 10 times, counting the number of times the front shaft turns.
If it is in North America I could use it in my '38 SS as mine is worn and noisy.