1.  Marriage Record:

2.  1790 Census Images (online at Ancestry.com, Image #4 of 8; extracted by Diana Gale Matthiesen):¤•

1790	NH	Rockingham Co.	Epping	David Laurence	p. 192	Ln. 12	1-1-2-0-0
1790	NH	Rockingham Co.	Epping	David Laurence junr	p. 192	Ln. 13	2-2-3-0-0

These data indicate:

Gender and Type	Age Class	Therefore Born	Individuals Inferred
2 free white males	16 or over	in or bef. 1774	= David (b. 1739/40) = David (b. 1771)
2 free white males	15 or under	1774-1790	= Joth./Jon. (b. 1777) = Samuel (b. 1779)
3 free white females	all ages	in or bef. 1790	= Lydia (b. 1745) = Sarah? (b. 1765) = Ruth (b. 1774)

Presumably, our subject is David, Jr. (II).  Joseph appears to be married and gone.  Sarah should really be married and gone by now.  Is there another, younger daughter we don't know about?  Or is the third female not a daughter?  The list is alphabetical, so no neighborhood reconstruction is possible.

3.  1800 Census Index/Images (online at Ancestry.com; Image #6 of 11; extracted by Diana Gale Matthiesen):¤•

1800	NH	Rockingham Co.	Epping	Roll 20 (Book 1)	p. 713	Ln. 08	Dav. Lawrence	10101-00101-00
1800	NH	Rockingham Co.	Epping	Roll 20 (Book 1)	p. 713	Ln. 09	Dav. Lawrence Jr.	00010-10100-00
1800	NH	Rockingham Co.	Epping	Roll 20 (Book 1)	p. 713	Ln. 15	Dav. Lawrence 3	00010-10100-00

These data indicate:

No. & Sex	Age Class	Therefore Born	Individuals Inferred
1 male	9 or under	1790-1800	= grandson
1 male	16-25	1774-1784	= son or son-in-law
1 male	45 or over	in or bef. 1755	= David (b. 1739/40)
1 female	16-25	1774-1784	= daughter or daughter-in-law
1 female	45 or over	in or bef. 1755	= Lydia (b. 1745)

It looks like our "Junior" (II) has become "Senior."  List was roughly alphabetized.

4.  Ancestors of Lydia Gerrish by Jane Leslie.

5.  Descendants of John Sias by Garey L. Marshall.