Quoted with minor amendment from Elite Series Vol. 1 U.S. Army Air Force:1 Development of the F-3 heated flying suit began in late 1943 and was standardized on 19 Feb 1944, It was a well designed 2 piece [ensemble] with an OD cotton/rayon twill jacket and bib-type trousers, and was intended to be worn over both thermal underwear and the standard wool service shirt and trousers, and could be worn under the B-15 jacket and A-15 trousers [both of which were made of brown leather lined with alpaca].
In combination, and with power supplied to the F-3, the combination was expected to protect wearers down to -60 degrees F (-50C).
Prior to 1944, there were less well developed suits creatively identified as the E-1, F-1 and F-2 heated flying suits. My refernce is short on pics of the intermediate suits, but the F-2 (standardized 8/43) is described as being a four piece unit, with electrically heated inserts that went into non-heated trousers and jacket. The electrical clothing was generally used as part of multiple layers, so crewman could have any number of a wide range of permutations. The F-2 and on had a chest level connector with a pigtailed cord that extended to the outer jacket, and allowed heated goggles to be plugged in there.
At first glance, the guys in Guppy's photo look like they're wearing just the inner suit; there is a visible cord but there's no visible thermostat (which arrived with the F-2A -- but I dont know what the placement was). The critical clue, though, is that the photo shows a 1 piece garment. That would mean the guys are wearing either the fairly early F-1 (24 volt, blue) or E-1 (12 volt, grey) -- which entered service all the way back to June 1941. These units had wire woven throughout the material, and could be opened as needed at the crotch without removing the entire garment.
Designers cliamed there was no shock risk to...um....critical components during ...um...relief operations, but I'd sure be nervous with an older suit.....
I think this is the first time my meager library delivered a requested detailed answer....