According to Graham Brown at Vintage and Classic Shock Absorbers, the lipped Hartfords for the J Types are 306s, not 506s which are the later versions without the lip.
It’s a small point, I know, but the gentleman whose company manufactured the Hartford shock absorber was T.B. Andre (no acute accent on the 'e') and Andre Hartford was not a person, or even a company, as is often imagined, but simply shorthand for Hartford shock absorbers made by T.B. Andre & Co Ltd.