I looked at JM’s book on p.199, VIN range from March to May 69 began with 61 and ended with 64. The VIN in all probability starts with one of those 2 digits, maybe even a 65. I looked on an L-78 registry but didn’t see a car listed with those possible VIN combos. The car may even be long gone or not on any registry.
I’ll try the tips mentioned to try and see the missing digits. I tried to post a picture of the stamp but it was too large.