Unfortunately, chalk markings were not codified and standardized, and were specific only to that train crew, or yard crews. Other than the obvious ones that are written in plain English, the specific meanings have been lost to time. Your speculations as to track numbers, door numbers, etc. are all valid, but in general, you can't look at a photo of a specific car and make anything more that a SWAG about the specific meanings of the individual marks. You'd need to talk to the person who wrote them, to decipher the "code" for that specific road/locale/crew.
Fortunately, we don't have to know what they mean for our modeling purposes, they just have to be plausible.
Tony Thompson has written several posts about chalk marks on his blog, and I recommend you read them as they are well-written and informative.