I'm not sure I agree that ROBIN should be considered a prototype. She raced at BYC and was sailing into the 1960's until we lost track of her. There are several sources that refer to her as the first. My guess is that the information written in the record is of variable accuracy, as has been proven the case on multiple occasions, so the exact LWL as recorded in the record might not be reliable.
The rudders, and a number of other characteristics, were different on the first 20, of which both ELF and ROBIN were members.
My question is: How does Ballentine know that ROBIN is different? From the builder's record or from first hand knowledge? Interesting that the last known location for ROBIN is Cataumet.