Well if it's a prewar Citroen car, I don't think it could be a Traction Avant, so that makes it pre-1934. If it is from the twenties that makes it either a Type A, B, or C. It has already been noted that it is quite small, so I'll go with the smallest type, and it was likely pretty old (I'm guessing an inexpensive old car, given the extensive modification.), so I will suggest an earlier date. How about a: Citroen Type B from about 1923?