It is an interesting definition. The language is not very clear.
Charelton (2015) - Distinguishes 3 water lines (WL) and 1 horizon line and says when polishing occurs, the horizon line disappears. Then defines Arnprior as 1.5 WL's with no trace of the bottom WL (no mention of horizon line). Their image on pg. 202 shows a slight change of slope on the island where the horizon would have been, suggesting that is the remnant of it. In addition, there is no mention of a 1.5 variety.
Haxby (2012) - Has Arnprior's differing by year, 1950/51 have 2.5 WL's and 1952 has 1.5 WL. The pictures on pg. 333 don't seem very reliable as some of them don't seem to match the description very well, but that could just be an element of the image/lighting.Does not seem to mention the horizon line, which would explain the 2.5 vs. 1.5 WL difference from Charelton.
Saskatoon Coin Club site http://www.saskatooncoinclub.ca/art...es.html#1950 - Has some nice clear images and follows similar definitions to Charelton. Their Arnprior shows the tails of the horizon line coming of the island and define it as 1.5 WL. The SWL is defined as having 3-4 WL's
Coin gets a bit tricky to identifyWhere this coin gets tricky, is the NGC grading. It is slabbed as a 1950 PL63 (no variety attribution). Charelton does not list any 1950 PL coins only MS and SP, Haxby lists MS, SP and PL. If it is Arnprior and charelton/coinsandcanada only list MS/SP, SWL has PL strikes in coinsandcanada.
PCGS's exmaple SP 63 coin shows a bit of the horizon line
https://www.PCGS.com/valueview/geor...=32445&h=pop as does the SP 67. Mine shows more/less depending on angle/light and I am not sure if some of the comes from reflection on the slab.
The coin itself looks closer to my 1951 PL dollar to me, a great glassy finish on both rev/obv, strong rims, etc... But I still do not have a good grasp of the 1950's/60's PL vs. SP differences.
I need to setup a proper photo setup to capture the details, my phone is just not doing it. Hopefully tonight or tomorrow. Thanks for the comments and looks so far.