That radiator is a true early flat top style, lots of people say they have a flat top rad when in fact they have the later version which is wider with angled ends on the rad top tank, wider rad may tie in with the later larger engines?
The rad should have a makers plate on it somewhere with the date it was made, normally made by Sercks.
Other early features are the bulkhead steady rods, front outriggers having no extra supports, floor dip switch blank fitted where lhd would have it fitted.
Lots of features are unique to early 58 models only, once 59 comes along they get more standard, then when the 2a comes out they are then much the same right up until end of s3 production, so early cars with early features still present are rare.