About the labels ...
I don't know of any significance to the round "PY" label. But perhaps we can deduce something, and also raise another question or two.
In Colin Mahoney's book, he sates, "A second rectangular label ... appeared when Harland production began" and for the small round label he said, "... the most common label ... on weights produced during the mid-1970s." The reference to "A second rectangular label" follows the section showing two versions of a rectangular CG label (for Caithness Glass).
So, in my understanding, Colin implies the rectangular PY label was the first in the 1970s period and the small round one came later in the mid-70s. This seems to suggest that the rectangular label was used 1971 to 1975, and the small round label from 1975 onwards [1975 being end of the first phase of the Harland works]. But we don't know for sure whether the small round label may have been used before 1975 and we also don't know if it replaced the rectangular one entirely. Similarly, we don't know if the small round label continued to be used during 1977 when Highland Paperweights Limited was set up and a large round "Highland Paper Weights" label was produced.
It is also interesting to note that in Anne Metcalfe's book "Paperweights of the 19th & 20th Centuries" [In the Miller's Collectors Guide series], there is an unfortunate lack of clarity in the Paul Ysart section. It was said that the small round PY label was "on Harland weights of 1970" [at the time it was believed by many that the Harland works started in 1970 rather than 1971]. It was then said that before 1970 a rectangular sticker was used. I have heard people interpret that as meaning the rectangular PY label (or sticker) was therefore used in the Caithness period – which as far as I know is untrue.