There should be neck dates and body dates in the neck pocket/neck heel. These would help lock down the date range of the bass. You can also try the G&L registry over at The G&L Enthusiasts Website
and see if a previous owner already registered it and noted the dates, etc in the registry. The early G&L's have "Patent Pending" on the pickup covers and the bridges; this in and of itself doesn't add any particular value outside of the value the earlier G&L's already command.
Pretty much an urban legend about Leo initialing neck pockets on G&L instruments * with the exception of the G&L Broadcaster guitar from 1985. These had a special label glued in the neck pocket and Leo's signature adorns these stickers.