Yep, all of the 3,426 M4A1(76) built had wet storage, as did later Shermans from that time (off the production line).
It could have been in service earlier, but the US Forces were reticent, it being a new and untried weapon, compared to the 'known' 75mm gun. Even as late as June 11th 1944, trials were still underway with the US Army in Britain.
When it was found that the serving Shermans, with the exception of the British-converted VC 'Firefly', were out-gunned, poorly armoured and outmanouvered by the German tanks, particularly the Panther, and later the Tiger, an urgent need for a more competitive gun was soon realised. The US had reservations about having a British solution, in the form of the 17 pdr on the 'Firefly', so took the M4A1(76). This allowed almost equal firepower, although not to the level of the Firefly, but manouverability and armour still did not allow parity with the German tanks, the main advantage coming from weight of numbers (of the Shermans) and a faster turret traverse than the opposition.
Forgot to add, later production versions of the M4A1(76) also had HVSS.