Mannequin heads (old doll heads)

Does any one have the specific requirements for measurements for baby sizes of head from newborn, 0-3 mos, 3-6 mos, 6-9 mos, 12-18mos 24 mos. I am trying to find doll heads in all these sizes. This is what I have found so far in vintage doll heads. I have a few other heads I use but they are not vintage and I like the look of the old much more.

I have taken 3 of these heads into stores and had them fitted with commercial bonnets to figure out the size of baby they would fit. from Left to right it is newborn, 0-3 mos, 3-6 mos and the last I have not sized yet it is either 6-9 mos or 9-12 mos. If any one knows the industry measurements for making baby garments for the head I would appreciate it or if you have some doll heads you do not want I promise they will go to a home that will love them and use them in nice ways as display mannequin heads for the bonnets I make. I like that these have different skin tones I would like to get some other skin tones as well to have a nice variety of models so I can have choices when taking photos for show and tell. After looking at these photos I realized one baby head was missing the one I showed with the white baby bonnet which is also a 0-3 mos size shown below.

Update:
Thank you those that left comments I have figured out that they are sized correctly and the last is 6-12 months which is what I figured it was.