USGA ruling On Use of the EOB Device--Practice and Play
The USGA ruling on the EOB Device rests on the fact that the device is composed of two pieces: 1) The flat base that may be epoxied in the rear cavity of a toe-heel weighted cavity back putter, and 2) the removeable hemispherical top.
The entire device may be used only in practice or in nonstipulated USGA rounds because it does not conform with Rule 1d in Appendix II of the Rules of Golf (USGA Decision 95-249).
However, if the hemispherical top is removed and the base is epoxied in the cavity and does not extend past the rear of the cavity, the putter CONFORMS WITH USGA RULES and may be used in stipulated rounds. (USGA Decision 95-302)
As can be seen below, even without the hemispherical top, the shading of the device and the reference line on the base still provide most of the advantages of the hemispherical top in alignment and execution of the stroke. If anyone should question you about this, you can cite the USGA ruling.
PUTTER SHOWING BASE OF EOB DEVICE WITH TOP REMOVED