Dear Murthy ji...
after a detailed review on the drug Nishpava (निष्पावा), would like to add a few more points here. As per the explanations in the Nighantu Bhava prakasha (BPN), we could find a few synonyms which denotes the drug based on its Morphology. Bhavamishra uses the terms Raajashimbi (राजशिम्बी) - which means the legume should be larger in size. He also uses the term Shwetha shimbi (श्वेत शिम्बी), which denotes that the legume should be white coloured or pale in its appearance.
keeping these names together in mind along with the key point that i could derive from Naveen ji's post which mentions the name in Kannada as Alasande a very commonly used vegetable here. The synonyms used in BPN very well goes with this vegetable Alasande as the fruits are extra large in size and the colour is pale green. Finally could derive the Botanical name for this plant to be Vigna unguiculata subsp. sesquipedalis. Sesquipedalis means one-and-a-half foot long. In English it is named as Yardlong bean (meaning the fruit measures one yard. This explanation helps to equate the plant to the synonym Rajashimbi mentioned in BPN. A link to the Wikipedia page has been given here for reference.
Also a photograph of the pale fruits of the above species could be seen in the following link which also specifies the name Alasande in Kannada in the description part.
Hope this post would be of some use to you Murthy ji and other friends.
Will post the photographs of the market available fruits later.
Thanks and regards