This should help
1975 K0 The frame serial number began GL1-1000012 while the engine serial number began GL1E-1000001.
1976 The frame serial number began GL1-2001483 while the engine serial number began GL1E-2001422.
1977 The frame serial number began GL1-3000001 while the engine serial number began GL1E-3000006
Good luck Prospect. The early naked Wangs are really nice bikes. I think all the other GW owners here will agree
Be sure to take some pics. The frame and engine vin just like the 750's won't match. On my 77 the last 3 of the frame vin is 169 and the eng vin is 208.
Before getting my 1977 GL1000 , just like you I was wanted a original first year Wing naked if possible. Thought I had found one had less than 30k on the dial but had fairing and bags added. The frame vin was correct for a 75 model year but after closer inspection the engine vin showed to that of a 77 model bike. That took the wind out of my sails and I let this bike pass by. Shortly after this I found my naked all original 77 with less than 10k. Sometime it pays to be patience.