The OED provides similar explanations to Merriam Webster, indicating that although over is the usual following preposition, others are possible. It shows examples from the time of Chaucer, and the older ones clearly indicate a variety of structures (on, upon, in, through as well as over).
As regards the 20th century, some, from 1922, and 1949 indicate a surviving use of pore with on and through as qualifying prepositions. Otherwise over seems to have become the only follower preposition in use with pore.
Note that the below extract only concerns sense 1b of pore. However at the end I have given two other senses (2 & 3), both stated to be obsolete (1788 latest), where pore seems to have been used without preposition. My supposition is that these uses have been superseded by peer. And we do peer into, peer through and peer at.
The OED gives the etymology of pore as "unknown", though it accepts it is probably related to peer (also spelled pire and pyre).
1b. To examine a book, map, etc., with fixed attention; to study or
read earnestly or with intense concentration; to be absorbed in
reading or study. Freq. with prepositions, esp. over.
▸c1387–95 Chaucer Canterbury Tales Prol. 185 What sholde he studie
and make hym seluen wood, Vpon a book in cloystre alwey to poure
[c1415 Lansd. powre].
▸c1449 R. Pecock Repressor (1860) 87 Thouȝ ȝe wolden labore and
powre and dote alle the daies of ȝoure lijf in the Bible aloon.
c1475 (▸c1399) Mum & Sothsegger (Cambr. Ll.4.14) (1936) Prol. 71
(MED), Þouȝ þat elde opyn it..And poure on it preuyly..It shulde not
apeire hem a peere.
1594 J. Lyly Mother Bombie i. iii. sig. B4v, In stead of poaring
on a booke, you shall holde the plough.
1610 P. Holland tr. W. Camden Brit. To Rdr. sig. 4, I haue poored
vpon many an old Rowle.
1675 W. Wycherley Country-wife i. 7 Mistresses are like Books; if
you pore on them too much, they doze you, and make you unfit for
1718 Free-thinker No. 37. 2 He rises by Three in the Morning to
pore over Mathematicks.
1771 J. Beattie Minstrel: Bk. 1st liii. 27 Where dark cold-hearted
sceptics, creeping, pore Through microscope of metaphysic lore.
1820 Keats Hyperion: a Fragm. ii, in Lamia & Other Poems 175 No,
no-where can unriddle, though I search, And pore on Nature's universal
1876 L. Stephen Hours in Libr. 2nd Ser. vii. 322 He had pored over
their pages till he knew them by heart.
1879 L. Stephen Hours in Libr. 3rd Ser. iii. 98 That disposition
which..delights in poring over its own morbid emotions.
1908 L. M. Montgomery Anne of Green Gables xxx. 345 I've pored
over that geometry until I know every proposition in the first book
off by heart.
1922 Times 24 Mar. 10/3 The old Flemish Bible on the pictures of
which he pored in childhood.
1949 C. P. Snow Time of Hope i. iv. 38 She pored with anxious
concentration through the advertisement columns of the local papers.
1991 G. Ehrlich Islands, Universe, Home iii. 28 We pore over maps,
chart our expeditions.
trans. To bring or put (oneself) into some state by poring. Chiefly in to pore one's eyes out: to blind oneself, ruin one's eyesight, or
tire one's eyes by close reading or overstudy. Obs.
intr. To look with half-shut eyes; to look closely, as a short-sighted person; to peer. Obs.