ADR plate will tell you the month, but the chassis number will tell you how early it is. Best bet is send me the ID details and i'll see what I can work out.
HJ Holden Sandman got black (19V) but HJ Kingswood Sandman got Slate black (18V). HX Holden Sandman spec was raised to Kingswood level so they got 18V. The only known HX Sandman with 19V is the red ute on the HX Sandman brochure. The reason it got 19V is because it was built during HJ Production prior to the decision to drop the base trim (ie Holden) Sandman for HX, and to build them all as Kingswood spec. They still used a Holden body and modded it to Kingswood spec as it was claimed to be too hard to not fit the side mouldings. If your Sandman is a really early one then it is possible to have 19V but it'd have to be from memory a 4/76 build which is when the 20 or so pilot HX's (all types eg sedan, wagon etc) were built. If it is a regular production HX (like 6/76 on) with 19V then either it isn't a Sandman or a mistake. Very unusual to see an error on a trim code though but it could still happen.