Lambros' list is correct for the most part except on IMs
1. Aluminum body (original)- fiberglass (replica)
(Many 356 lookers feel under the fender well lips to make a quick determination for glass as most replicas have a flat edge from the mold. IM has a rolled lip as an option to giver the edges a rolled factory metal effect)
2. Bolt pattern. Most replicas use 4 lug VW wheels(Not always).
Original use the wide five pattern,or 5x120 pattern.
(Most IMs use a Wide Five Disc front & rear. The CB Wide 5s are close to the original look. Using Drum Skinz will also give the original look))
3. Hand brake on original is the umbrella style under the dash. Replicas have the "regular" hand brake position between seats.
(An IM option is having the Umbrella Style E Brake under the dash.)
4. Pedals come from the floor on originals,replicas have the VW style.
(IMs come from the floor with 356 Pedal covers)
Additional - IM has a square tunnel on the floor that replcates the original and uses a 356 style shifter. Also, some IM owners opt for an original steering column (see Dale's). Most use original bumper guards, bumper rubber, side trim, rocker deco rubber, etc. The dash padding more resembles an original and the footwell area is wider, replicating an original. Side entrance door rubber is original repro and the glassed inner front wells resemble an original. The big give away on an IM are the roll up (power) window channels.
Both the High Bow and Low Bow tops have inner liners, giving a more authentic look. IM also has optional High Riders *American Bumpers*, various styles of bumper guards to fit the year replicated, optional Beehives for a pre-A or early T-1 appearance.
FWIW Gee Whiz Info only. (NO Sales pitch)
Jim