They were a regular production model in the first year. Ruger won't release production numbers (yet) but we est. that only about 20%-25% of the fixed sight, sq. butt guns were made in .38 spl. In the first couple of years, I'd guess that, by far, most of the production was adj. sight models. So, let's say, only about 20% may have been fixed sight, sq. butt (and another 20% fixed sight, round butt). In later years Ruger went after the LE market in a big way. Most LE contracts were for fixed sight models.
The grip frame was changed about the time the ser. no. prefix changed from 150- to 151-. Therefore, the early low back grip was used for only about the first 100K guns. Working the precentages back results in the 'very rare'. Plus, they just don't seem to show up very often.
Hope this helps,
Terry T