Cell-type specific methylation profiles: original reference finding?
Note: This is not my field, and only the result from a quick  database search



Doing a quick search in the Web of Knowledge, I found the paper Ehrlich et al (1982), Amount and distribution of 5-methylcytosine in human DNA from different types of tissues of cells, which is relatively well-cited. This in turn mentions that: 


  Recent studies indicate that for many genes associated with vertebrate development, there are tissue- or cell-specific differences in the extent of DNA methylation.


The oldest studies referenced in this paper that seem to refer to tissue-specific methylation are Vanyushin et al (1970), Rare Bases in Animal DNA, Nature and Vanyushin et al (1973), The content of 5-methylcytosine in animal DNA: The species and tissue specificity, Biochimica et Biophysica Acta. The 1973 paper is also one of the earliest papers cited in another well-cited paper from 1983 (Gama-Sosa et al, 1983)), so it might be the best bet. However, Gama-Sosa et al also mentions Kappler (1971), The 5-methylcytosine content of DNA: Tissue specificity, which is clearly relevant. In total, Gama-Sosa et al has references to 13 early papers on DNa methylation, so you might want to check them all out, and backtrack from there. Also, Vanyushin seems to have published in smaller, more obscure journals around the same time (e.g. DOKLADY AKADEMII NAUK SSSR), so there might be earlier papers that express the same ideas. The earlier papers seems to deal with bacterial DNA methylation though.

A couple of other references are:


Van der Ploeg &amp; Flavell (1980), DNA methylation in the human gamma delta beta-globin locus in erythroid and nonerythroid tissues, 
Mandel &amp; Chambon (1981) DNA methylation: organ specific variations in the methylation pattern within and around ovalbumin and other chicken genes
McGhee &amp; Ginder (1981), Specific DNA methylation sites in the vicinity of the chicken beta-globin genes