1970-76 spare tire tub differences

[Broman]

Regarding the deep (full-size) spare tire tubs, I have seen two differences.

1) Some have one drain hole (fwd) and some have 2 (fwd and aft).

2) Some have what looks like a heat shielded welded to the side next to muffler.

The question: what is the date of these changes?  Before I weld in my salvaged tire tub I wanna make sure I have the correct one.

[Paulytunes]

This is mine (build date of 7/76):

One forward plug and one aft plug.