Krish, I'm not sure I understand your question. First, both verbs you mentioned use past participle. The difference is "were reproduced" needs auxiliary verb "were" because "statues" cannot perform the action of reproducing but are the recipient of it: They are being
I think your question is about whether there should have been the presence of a past perfect tense for "had been reproduced" because it happened before "yielded". Well, I don't think we require past perfect because we are talking about 2 different actions to 2 different subjects.
1- The excavation
2- The statues
The past perfect tense would be used if we had wanted to distinguish between the different time frames happening to the same subject.
Ex: The excavation had been a waste of resources for many years before it yielded any result.
In the above, you can see that before
the yielding of any result, the same subject
, the excavation, was a waste of resources. Thus, past perfect is useful to separate 2 actions performed by the same subject, or to the same object, in the past where one action comes before the other.