Good score. Nice clean 'low back'.
By the way, all 'low back' fixed sight square butt guns were marked 'Security Six'. The 'Police Service Six' marking didn't come into use until the 'high back' frame at about ser. no. 151-000x. (there is some over lap of ser. no.).
Of course, we're always on the lookout for a low back marked with the service six logo or a high back marked with the security six logo. We have found about a doz. speed sixes marked with the service six logo.
It has been reported that a few (maybe a doz.) 6" fixed sight guns were made, we've not turned one up yet in this collector circle.
Terry T