I am pretty sure that all of the factory-original Model 27's were built on square butt frames.

The first production round butt N frame from S&W was the 3 inch M-24-3 .44 Special. 5,000 made in 1983 for Lew Horton.

There were a very few round butt N frames that were not authorized but were 'snuck through' the factory in under-the-table deals, like Doc 44's fixed sight, M-58-based .44 Magnum, but these won't letter and would be difficult to prove they were made that way at S&W.