There were stainless 150 prefix lowback Security Sixes in addition to the 151's. My lowest is a real outlier, a GA32 at 150-55384 which must have been from the very first experiments with stainless and it received a high polish gloss finish at some point before leaving the factory extremely late in 2009 during a warehouse cleanout. Probably a prototype that sat around the shop until they finally put it in a 1980's style yellow box and wrote "High Polish" on the end in ballpoint (then when they shipped it out, they slapped an orange barcode label on that end partially obscuring the note). I always thought it may have had a standard finish until they picked it as a test subject to try the high polish treatment for the glossy Vaquero which was Ruger's first glossy stainless offering, simulating nickel plate.
But from regular production, my next lowest stainless lowback is 150-92573, also a GA32 but with regular finish, and 150-95114 which is a GA34.
And like you, I have blue highback Police Service Sixes in the upper 150 prefix at 150-94446 (SDA84, bought new by my dad), 150-97027 (SDA32), and 150-97331 (SDA34).
I have another interesting pair from that transitional period, lowback fixed sight blue guns in the 151 prefix that are rollmarked as Security Six rather than Police Service Six. That further strengthens my opinion that only the highback guns were marked Police Service Six. They are different calibers but are very close together SN-wise at 151-01438 (SDA84) and 151-01445 (SDA34).