94 bird a/c fitting thread size?

got a 94 thunderbird,want to convert the a/c to 134a.took the fitting off the low port side and can't get a conversion fitting to fit it.is this a wierd uncommon fitting.i have tied two different brand replacement fitting assortments and no luck.the threads are really rusty and i want to chase them but cant find the correct thread size.anyone know?

Send an email to Donald91tbird as he just made this conversion and he might be able to tell you what fittings to use. See his post "A quick question - need to comfirm (A/C) ". His post is at the bottom of page 3 or the top og page 4.