Adam lay i-bowndyn
The text you are citing, commonly known as Adam lay ybounden, is a text from the 1400s preserved in a single specimen. The original text was not spelled as you have it. Rather, it was written like this:
Adam lay i-bowndyn,
bowndyn in a bond,
Fowre thowsand wynter
thowt he not to long
And al was for an appil,
an appil that he tok.
As clerkes fyndyn wretyn
in here book.
It doesn’t really have a “title” per se; rather, as is oft done, it is known by its first line.
Yes, fynden is not a past participle. It might be a present plural which would be the same as the infinitive, and it might just possibly be a past tense plural. In any event, it matches number with clerks. In Modern English, that would be
“And all was for an apple,
an apple that he took
as clerks find (or found)
written in their book.”
(Sorry, but modernizing it kills the scansion.)
There were regular base forms of the verb like FYNDEN and FINDIN, which would be the same in the infinitive as in present plural just as today. So perhaps this is just another spelling of one of those two.
The quality of the vowels suggest it was not a past tense, although there was a FYNDON in the 12ᵗʰ century; as a macaronic text, it might grab that, although I’m not convinced. The OED lists the past tense plural forms of find as:
(9 dial. FANT).
As you can see, there were a lot of ways to go out at it, and given the macaronic nature of this text, I would not put too much in finding one form one place and another form another place. It’s made up of many stray bits.
Past participles beginning with y- are archaisms left over from Middle English and Old English. They are cognate with modern German participles like gebunden. You can find the word written in a more German way in Beowulf:
Cyninges þeʒn··word oþer fand soðe ʒebunden.
The OED’s last two citations for ybound are:
- 1647 H. More Song of Soul ɪɪ. Democr. Plat. ᴠɪɪɪ, ― The low Cusp’s a figure circular, Whose compasse is ybound, but centre’s every where.
- 1714 Gay Sheph. Week Prol. 84 ― Thy joyous Madrigals twice three, With Preface meet, and Notes profound, Imprinted fair, and well y‐bound.
Why was it not ʒewriten as in Old English or ywriten or ywryten as in Middle English? In part because the y- forms died out at different times in different places, but also because here the scansion seems to call for it. It might also be an intensifying version of the past participle.
There was some restoration of y- forms like yclad and yclept as deliberate archaisms by some. The OED writes:
The general facts of the history and survival of OE. ʒe-, of which some details are given below, are :– In positions where it was still recognizable as a prefix, it had left few traces in northern English by 1200; its disappearance in the north was assisted by the absence of the prefix in ON. Substantival, adjectival, and verbal forms (other than pa. pples.) continued, not later than the end of the 14th century, only in southern and west-midland dialects. The pa. pple. was regularly formed with the prefix in southern ME. till about the middle of the 15th century, and its use in the form a- survives in south-western dialects to the present day. Pa. pples. so formed were a prominent feature of the archaistic language of Spenser and his imitators, and a few of them, the most notable of which is yclept, persist as conventional archaisms of poetry.
So now and then you will see these used even today by archaizing writers, mostly poets.
What many people don’t know is that we still use its remains today without noticing them or thinking about it. For example, handiwork decomposes to hand + i-work, and was written handʒeweorc in Old English. There are several other examples like that, such as how close enough really used to be to modern German genug back when it was written ʒenoh, or how aware was once ʒewær.