What are the characters in this seal script inscription on an ancestral altar? (Characters identified: 孝思維則)
as @drooze revealed, it's &quot;孝思維則&quot;
reading from right to left:
孝: http://humanum.arts.cuhk.edu.hk/Lexis/lexi-mf/search.php?word=孝
思: http://humanum.arts.cuhk.edu.hk/Lexis/lexi-mf/search.php?word=思
維: http://humanum.arts.cuhk.edu.hk/Lexis/lexi-mf/search.php?word=維
則: http://humanum.arts.cuhk.edu.hk/Lexis/lexi-mf/search.php?word=則
the verse is original from 詩經　大雅　文王之什　下武:

成王之孚
下土之式
永言孝思
孝思維則

the ctext.org has a translation:

He secured the confidence due to a king,
And became a pattern of all below him.
Ever thinking how to be filial,
His filial mind was the model [which he supplied].

https://ctext.org/book-of-poetry/xia-wu