Ok, we'll need to investigate further:
Homologation form 3028: group 3 in 1970 (If I look at appendix J of 1969, 1971 wouldn't have been written at that point) = GT car
Homologation form 600: group 4 in 1970 = sportscar
So we want to use homologation 3028 if we want to race as a GT car. This (or the other) homologation form would automatically put us in category G2 (1/1/1970-31/12/1971) because the homologation would only become valid on 1/1/1970. If you would look at Appendix J of 1971 you see that Group 3 is production GT and group 4 is GTS, however I think we need to look at the earlier one.
Appendix K:
Touring, Competition Touring, GT and GTS cars of Period
G2 (1/1/1970-31/12/1971) onwards must comply with the
Appendix J international Competition regulations appropriate to
the final year of their period as defined in Article 3.
So for us that's Appendix J of 1971.
However, I also found the part that you were referring to:
(b) These models must have a competition history and the special
mechanical parts are limited to those authorised in the period
Appendix J rules applicable to Group 3 cars and Article 5 (Safety).
(c) All modifications must respect the technology of the period as
applied to that model.
(d) Should these models have been subsequently homologated
by the FIA in Group 3, they may only use those mechanical
parts included in the relevant extension of the FIA Group 3
Homologation Form.
Does this "subsequently" mean that if a car has been homologated outside of the period, so let's say last year on bases of photographs (like Stefan's car)? It's not a very clear paragraph.
Also, the part about the "alternative components" is very confusing:
Alternative components to the original manufacturer’s specifications
can only be used if it is proven that these components were allowed
by the period Appendix J and/or homologation form applicable to
the car concerned and, in both cases, used in that model of car in
a Competition entered on the FIA International Sporting Calendar in
the period.
I will also ask some questions on the 10/10ths forum, they will have some more answers.
Serge