Attached are some sources that I have assembled. [See <http://www.aishdas.org/avodah/faxes/fingerEsrogSources.pdf> -micha] And a nice picture of Rav Machpud examining a finger esrog: [<http://www.aishdas.org/avodah/faxes/fingerEsrogRavMachput.jpg> -micha]