Yes these were used in vehicles so the hammer didnt catch and let a round of the the tight compartments of a vehicle, i believe yours is the Enfield Mk1* or Mk1** Service Revolver, i have a Mk1** of this type but yours is in much better condition, These were only produced in war-time and after the war were declared obsolete, i'm not 100% sure when these started production but the Mk1** started in 1942. They were designed as a speedy production at first as there were less components but there were safety concerns about them, then the tankers took a liking to them and started using them. Some early ones were converted revolvers with the complete hammer but the ones with the plastic grips are usually Mk1**'s and not converted.
I may not be correct but on the barrel facing upwards there should be (we there is on mine) 2 numbers, mine is "43" and I've always assumed that is the date
See "Variants" on this page: http://en.wikipedia.org/wiki/Enfield_revolver