I think the marking on the lens is more likely to be right than the invoice.As Michael says; the only accurate way to measure the focal length is to find the position of the rear nodal plane of the lens. However, you may be able to verify the focal length from the aperture scale, because some process lenses have two scales for the aperture. One scale is the usual f-number marking, and the other is the actual aperture diameter, marked in millimetres.A 360 mm f/9 lens will have a maximum aperture diameter of 40 mm, and a 305 will have a diameter of 33.9mm. At f/32, the corresponding diameters will be 11.25mm and 9.5mm - enough of a difference to distinguish them - if that second millimetre scale is there.<b>In any case, you could measure the diameter of the front element with a pair of calipers. With this simple design of lens, the front glass diameter will be close to the theoretical maximum aperture size. If the front glass is around 34 mm diameter, then you know you've got a 305mm lens. If it's 40mm or more, then there's a good chance the lens is 360mm in focal length.

I'm betting you've been fobbed off with a 305mmm lens, BTW.