In a January 1960 issue of Skin Diver I found a full page ad for Sportsways Waterlung regulators. At the bottom of the ad was a drawing of an SPG. The text reads:
"For the first time the High Pressure Reading Gauge...A VITAL SAFETY FACTOR...becomes a useful and practical component. The Sea Vue Gauge can be purchased separately and installed in a few moment
No price is given, but the same ad indicates that it can be purchased along with the Waterlung Model B for $90. The Model B without the Sea-Vue gauge is listed at $65. The gauge face, not shown in the ad, is perpendicular to the hose, and clearly screws into a first stage port.
In the same issue a company called Mar-Mac has an ad for, among other things, a " Se Air Universal Lung Gauge". It does not look the same as the Sportsways Sea Vue, in that the gauge face (also not shown) on the Mar- Mac is parallel to the hose, and the other end of the hose is not threaded. How and where it attaches is unclear. A small circular contact point with an O ring is suggested by the drawing.
While looking through 10 Skin Diver magazines from that era I did not find any mention of these devices in any articles.