The only significant difference between the Landeron Electric calibres 4750, 4751 and 4670, is the hacking mechanism.
By the way, “hacking” is the process of stopping the watch when the crown is pulled out; it is often regarded as essential when accurately setting a watch because it allows you to precisely set the second hand. With electric watches, a good hacking mechanism not only stops the watch but also disconnects the battery from the circuit e.g. 214 Accutrons have a poor hacking mechanism in my opinion, because the tuning fork continues to run even with the back set crown pulled out.
Firstly, the 4750. It has no hacking mechanism; the watch and hands continue to run even with the crown pulled out. The end of the stem is visible in the top part of the image; it has the hand setting cog on the end of it:
Secondly, the rare 4751. It has a hacking mechanism but no battery disconnect. The hand setting lever now has an extra arm on the right hand side; this moves over the top of the balance when the crown is pulled out. A small brass peg on the balance periphery is caught by the arm extension and the balance stops; the peg can just bee seen at the very bottom of the photograph:
Lastly, the 4760. It has a hacking mechanism and a battery disconnect. The hand setting lever has gone back to the 4750 style but two extra levers have been added. The first one (steel coloured) is engaged with the end of the stem and operates on the second one (brass coloured). The second lever has a nylon sleeve on one end that pushes against the side of the balance once the crown is pulled out. At the same time, this second lever moves away from the vertical brass post and therefore breaks the circuit: