Mik, under the cowl is usually over spray body color and under the cowl panel is where the black satin paint starts. The firewall, inner fender wells, core support and attaching parts are all in the satin black.
There are a few hidden partial vin stampings on the car... I think another behind the heater cover from the firewall side etc... Maybe Jim knows just why they were stamped... I've never heard a definitive answer on that.
But depending on how original you are going, you could paint the whole area under the cowl black, or you could have some over spray. Personally I like it clean looking, all black and finished nicely.