The objective slider has a female dovetail sliding into the male dovetail on the microscope body. This differs from later models in which the objective slider was male. A sprung ball bearing(red arrow) on one side near the front of the body registers each objective slider via indentations(red arrows) on the underside of the slider. There are three objectives including the low power 16mm, and what is likely a 40X and lastly, a high power oil immersion objective. The 16mm objective is free to rotate up and down via its threads; the other two objectives have lock nuts to hold them in position.