Just adding to the general discussion.
Do have examples of numerous (not just Y2 to G2) other radiator top tanks being relabeled with the small letter tag over the stamped one. Looking at pictures and notes Y over a G, Z over some unknown letter, Z over a G, Y over some unknown letter and others.
Have pictures of examples where the relabeled radiator top tank was dated to match the cars projected and documented production period. Also these fall within a group and within a two month period or grouping of cars produced at San Jose suggesting IMHO that they were installed originally. Would not suggest that someone having difficulty finding a correct stamped radiator go out and relabel another since this does not appear to have taken place during the whole production year. Might dive into this deeper elsewhere rather than get this thread too far off thread