After I posted the reply to you, I myself googled 'on a/- par with' and found the result you are showing. I am really embarrassed with it. OED gives only 'on a par with'
1. a. Equality of value or standing; an equal footing, a level. Now chiefly in phr. on or upon a par.
1662 Petty Taxes 26 A natural par between land and labour. 1672 I Pol. Anat. (1691) 63