A few points. The underwater speaking device actually sort of worked over short distances, but was a pain to use since you had to remove your reg (I had one). The sound was muffled but generally understandable. They worked by transferring the air vibs to the water through the "ballon". Depth guages WERE common in the 70’s. The cheap ones were capillary tubes with index marks. They were very accurate in shallow depths, harder to read deeper. Bourdon tube (modern mechanical) have been around for quite some time and the design dates to the mid 1800’s and is essentially the same device as your TPG. The BC was an early jacket style from late 70’s (a bit fuzzy there). They often had CO2 inflators which sucked if they were accidentally fired at moderate to shallow depths. At least it’s not the "horse collar" style. The deco gauge was an early mechanical dive comp. Actually, its an analog computer of sorts. It attempted to integrate time vs. depth and estimate the nitrogen load (probably based on the old Navy tables). I’ve seen an earlier model that looked like a needle and graph. aka the bend-o-matic.