For a Swedish user, it is worth noting that the most useful Sweden-based source is The Little Sindy Museum, which is operated in Sweden under license from Pedigree and reproduces many catalog images and doll photos with Pedigree's permission. 

I have prioritized sources in the following order: official or licensed Pedigree sources, especially Pedigree's own current website and The Little Sindy Museum; then large, well-established collector references such as Our Sindy Museum, then original catalog references, identification guides, and finally auction/sale pages with clear close-ups when required to capture accurate text or late license marks. This prioritization is important because some late markings, especially those from 1986 and the 1990s, in practice survive best in photo-dense sales pages rather than in official documentation.

The chart summarizes the year of introduction or earliest recorded use of the major mark groups. For "2 GEN 1077" I have chosen to show both 1978** and 1979, because Our Sindy Museum links the mark to the North American launch under Marx in September 1978, while the same site ties the broad Pedigree rollout to the 1979 catalog re-organization; simpler collector guides sometimes date it more broadly to around 1977–1980.

If dating an unknown Sindy solely by marking, the safest rule of thumb is as follows: country text without number code usually points to the 1960s; 033029/033030/033055X/EG 033087 X points to the early 1970s recast; 033050X belongs to the mid-1970s; 2 GEN 1077 / 033055X is late 1970s to early 1980s; and Sindy over 033055X is typically 1982–1985. 1986 breaks the pattern with the new “Smirky” head, where 42103-001 appears in secondary but repeated documentation

The most difficult dating case is the transition between 2nd Gen and Sad Face. Some guides place 2 GEN 1077 as early as 1977, but the stronger catalog-linked sources indicate that the marking first certainly appears in the Marx launch in North America in 1978 and then becomes a broad Pedigree standard in the 1979** redesign. At the same time, the 1982 sources show that new Sad Face heads and older 2nd Gen heads could appear in parallel in the same fashion year. This means that a 2 GEN marked doll in 1982 clothing is not automatically “wrong”; it may be completely original in the box.

Another important detail is that the same text can appear in several production phases but on different bodies. For example, "Made In Hong Kong” is seen on both early 1960s HK bodies and on some mid/late 1970s Pedigree-era bodies. That is why the text must be read in conjunction with body shape, material and joint construction. TLSM's own body comparisons for 1976 and 1977 show just this: the location of the Hong Kong marking is similar, but the rest of the body is clearly different.

There are also documented anomalies. For example, Jenjoy shows a doll with a head that looks like an early Hong Kong-made doll but is still marked Made In England, while the body is marked Made In Hong Kong. This is a strong indication that at least some molds, heads, or assembly flows were mixed between factories or subcontractors. Such anomalies should be considered true production variations rather than automatic signs that the head has been later changed.

For the later licensing periods, the markings are generally better as manufacturer identification than as exact year identification. Hasbro bodies with Hasbro Inc. on the lower back certainly say "Hasbro era”, but you still need face shape, eyes, body type and often outfit collection to determine whether the doll belongs to 1988, 1991, 1993 or 1994. The same goes for Vivid and New Moons: the markings help, but the visual differences still carry a large part of the dating.

There are still some marking groups where the evidence is incomplete. The most important one concerns 1986 Pedigree heads: I consider 42103-001 to be the best-attested numerical marking for the new “Smirky” head, but it is even more clearly attested in sales pages and secondary image sources than in official factory documentation. Therefore, that line is marked medium-certain and should preferably be double-checked against a proven original boxed copy with clear neck photos.

The early 2000s are also somewhat murky. For New Moons / Hachette / nearby re-releases the forms “©2002 PT & D” and “2002 PD&T” reappear, but I could not definitively determine whether the difference is due to actual mold variants, misinterpretation in sales pages, or how the marking is read in the image. I also could not confirm a completely secure, uniform placement for these marks. 

Finally, I have not included every possible license or market variant unless I found strong enough evidence for the marking. This includes more peripheral or late licensing such as Funskool, Giochi Preziosi, some Chad Valley releases, the Tesco line, and some collector's editions where I could not verify the actual doll marking with the same certainty as for Pedigree, Marx, Hasbro, Otto Simon, Vivid, New Moons, and Kid Kreations. Pedigree's official modern 60th anniversary doll, however, is securely marked as individually numbered via a separate card “X of 1,963”— but that is a packaging/accessory marking, not a secure body stamp on the doll itself.