C. Bernoulli, 'Stierprotomen und ihre Rätsel', Intuition und Kunstwissenschaft: Festschrift Hanns Swarzenski, Berlin, 1973, pp. 45-52.
C. Bernoulli, op.cit. notes the numerous influences between the Christian religion and the Eastern worship of Mithras, in which the bull was the predominant symbol, standing for fertility and life. The use of these rings is discussed and it is suggested that they are of more practical use such as harness rings, rather than symbolic or relating to a cult.