Do you know why ow words such as town, growl, crowd, that have the /ou/ sound can only be followed by an l, b, or d? Is this an Anglo-Saxon convention?
First, concerning the distribution of the <ou> and <ow>:
In addition to <l, b, d>, <ow> also occurs before word-final <n>: gown, brown etc. (But <ou> occurs before <n> only in word-medial position: bounce, council, lounge, etc.)
The only instances I can find where <ow> spells /ou/ before <b> are in compounds in which the <ow> is at the end of the first base and the <b> is the first letter in the second, as in browbeaten, cowbell, cowboy, howbeit, plowboy, sowbelly. In the great majority of cases <ow> in this context spells long <o>: blowback, crowbar, lowbrow, rowboat, shadowboxing, showbiz, snowball, throwback, towboat, yellowbird.
In American English Spelling I try to summarize the distribution of the <ow> and <ou> spellings of /ou/ as follows: The distributions of the two are fairly predictable, though a bit inelegant to describe. The correspondence /ou/=<ow> is regular (1) in word-final position, (2) before vowels, and (3) before <l> and word-final <n>. There is overlap between <ou> and <ow> before medial /d/ and before /z/. Everywhere else /ou/ is spelled <ou> (p. 303).
Second, as to why this distribution:
Many (maybe all) of the <ow> spellings developed quite late 14th or 15th centuries so it seems unlikely the distribution of <ow> spellings is due to any Anglo-Saxon conventions. I think the distribution is due simply to contingency in the history of English sounds-and-spellings. The lists of earlier spellings in the Oxford English Dictionary reveal considerable indecision about the use of <ou> and <ow>. For instance, from the 16th to the 18th century crowd was spelled with <ou>, and <ow> did not appear until the 18th century. Some earlier spellings of howl: houle, whoule, howle. Of brown: broun, broune. Some of the indecision could also be due to the fact that <w> was created in English in the 7th century in the form <uu>, as the name double-u indicates.