The SV guns were manufactured from very late in 1944 through early 1946. They are a part of the "second million" M&P production that began in 1942 with a V prefix -- the Victory models. After a sailor was killed in late 1944 when a dropped gun discharged despite the presence of a hammer block safety, the safety was redesigned and guns produced after that date also had an S in the prefix to indicate the presence of the new safety block. Many predecessor guns were retrofitted with the new safety block, and an S stamp was added to those guns on either butt or sideplate, or both.
New production safety block guns with the SV prefix are numbered from about SV730000 to about SV815000 (very round numbers; allow for some variation). After that the V was deleted, and the remaining units of the "second million" were produced with just the S prefix until 1948, when the C prefix was chosen to designate M&Ps produced in the third million.
I haven't yet come across an SV revolver, but hope to. One of those is bound to be a birthday gun for me.