There are a few possibilities here, depending on exactly what the team used in 2018, and whether/how the officials interpret conventions that are no longer part of the rules. The safe bet is to just say no, but if your team is willing to ask the Q&A and/or take some risks with inspection, there is some room for interpretation.
In particular, R15 relates to “elements created before Kickoff”, but the creation of an item is not defined, and the agents acting to create the item are not identified. Notwithstanding the history of parts utilization rules in previous years, this is an ambiguity that could lead to some difficulties with enforcement, or some opportunities for you.
I suspect that what FIRST intends by R15 is that your team may not use fabricated items whose fabrication took place before Kickoff, and was done by anybody acting on behalf of or with intent to benefit a strict subset of teams (your team is an example of a strict subset of teams). Absent such a clarification (you could ask in the Q&A, and FIRST could issue an update), you could raise the argument at inspection…though I can’t say I’m optimistic about your chances of success.
Coming now to the blue box (which is advisory, not mandatory), I don’t think it corresponds to the plain meaning of R15—and this is part of the reason I proposed the extrapolated meaning as a possible solution. I can’t see a way to reach the conclusion FIRST has reached without assuming facts not in evidence in R15.