I'd say that the complexity of the joints (shape, and number, of the panels being joined) meant that they were likely to be - as we have seen in the 'nude' shots - 'cosmetically challenged'. Add a little assembly worker variation into the mix for good measure. They required filling/smoothing, and the top of the rear quarter joint was a large area. I can't think of any other areas of the early S30-series Zs where cosmetic filling was required before paint, as I pointed out up-thread. Can anyone think of any?
This is specifically a Nissan Shatai thing. If it was their habit to use lead, and they had workers who were skilled at applying it well, and fast, then why not? It certainly 'dries' and is workable much faster than a plastic-based filler and there's no use-it-or-lose-it time constraint as there is with a hardener-added mix. Much better for a busy production line. And as we know, the lead is very resistant to cracking. Seems ideal.