Smith never manufactured a Model 66 (or 19) with a 7-shot cylinder. They're just not hefty enough. Only the slightly larger L frames (686) could accommodate the seventh hole in the cylinder.
On a side note, the 66/19's are great guns and although not in production, can be found used. Lots of them were made - especially the 19's.
I load 9mm, 10mm, 38 Short Colt, 38 Spl, 357 Mag, 44 Spl, 44 Mag, & 45 ACP.