The Dorfsman 'a'

Following on from our Lou Dorfsman post, someone asked what font the 'ha ha ha' was in. See the lower case 'a' above. Well this is a mystery to us here at 'Things to Look at'. Did he redraw it specially to work at that size? It seems likely that the extreme contrast between thick and thin lines would disintegrate at smaller sizes making specific size versions a must. We know that he created CBS Didot and CBS Sans and we are aware he was a fastidious man making it seem highly likely that this is a one off hand drawn letter.

Could it be an existing font? A lost font perhaps? Or is there some other explanation? If anyone knows the answer then it would be very interesting to know. All we can work out is that is none of the usual 'modern' suspects shown below, especially as many of them post date the poster which was created in 1961.