Im glad to read this as its proved the old grey matter is not so bad after all
When I worked on the 505 book we had also worked tiredly on a bigger book covering all the 500 series range, sadly that book never came about due to unforeseen circumstances. It would of been a 300 page book and the printing cost was rather higher than first thought.
In that book was all the info needed about all the other models like the 501 503 506 506M 507/8 so when I said I thought the 506 came out in 1969 i wasn't a million miles away
Another part was I actually have the Tight Lines book somewhere so will search it out instead of damaging my collection of catalogs.