It still doesnt explain why Thomas birth wasnt registered yet the other siblings were.
Not sure which Thomas you are referring to?
If William & Allison Halbert who had children baptised in Castle Garth Chapel are the same couple who married in Kelso in 1763, then there are a number of years in which they could have had children who do not yet show up in baptisms in Newcastle (they may be somewhere we haven't looked yet, or they have may been baptised outside Newcastle).
We know from John Potts Halbert born 1768, & some of the burials that Janis found in her index for the Ballast Hills burial ground, that William & Allison Potts had at least a couple of children prior to the Castle Garth baptisms.
We know from William Halbert senior's will that William & Allison had the following surviving children at the time the will was written:
John
William
Joseph
George
Elizabeth
Alice
Isabella
I have pulled the following list of children together - if you have more, can you plse slot them in so we know where we are. William (listed above) must fit in somewhere between 1763 and 1770, I think.
Children
Elizabeth daur of William and Allison Halbert died 27 July 1771, aged 5 years
[so born about 1766]
John Potts - born about 1768 - see his marriage licence and we know he died in Edinburgh in 1826 aged 58 (see earlier posts)
George s/o Wm & Alison Halbert bap 2 June 1771
Baptisms, Castle Garth Chapel - Family SearchGeorge, son of the above died 10 Dec 1773, aged 18 months
Burials, Ballast HillsThis burial date seems too late to me (needs to be checked), as another George Halbert, s/o William & Alice was baptised in Castle Garth Chapel 21 March 1773 and this would fit with the following death:
Gateshead - 14th inst [July 1822] at the Windmill Hills aged 49, Mr George Halbert formerly of Newcastle
Durham Advertiser, 27 July 1822Elizabeth bap 19 Mar 1775 d/o William & Aley Halbert
Baptisms, Castle Garth Chapel - Family Search On Sat last at St Nicholas’ Church, Mr Matthew Corner, of Gateshead, builder, to Miss
Halbert, daughter of the late Mr W Halbert, of the Castle Garth, in this town, draper
Newcastle Courant 22 June 1805 Elizabeth Halbert of this parish married Matthew Corner, Gateshead on 15 June 1805
Marriages, St Nicholas, Newcastle upon TyneJoseph, son of William and Allison Halbert died Oct 4 1776, in infancy;
Ballast Hills, BurialsAlice d/o William & Alice Halbert bap 27 Feb 1778
Baptisms, Castle Garth Chapel - Family Search Possibly?
Alice Halbert of this parish married Edward Aikine Davidson of the chapelry of St John on 16
May 1804, witnesses John Thompson & Thomas Brown (both frequent witnesses)
St Nicholas Marriages, Newcastle upon TyneJoseph s/o William & Alice Halbert bap 2 Jan 1780
Baptisms, Castle Garth Chapel - Family SearchRobert s/o William & Alice Halbert bap 18 Nov 1781
Baptisms, Castle Garth Chapel - Family SearchRobert, son of the above, died March 30 1786, aged 4 years and 5 months;
Burials, Ballast HillsIsabella d/o William & Alice Halbert bap 2 Apr 1783
Baptisms, Castle Garth Chapel - Family SearchAnn d/o William & Alice Halbert bap 20 Nov 1785
Baptisms, Castle Garth Chapel - Family SearchAnna, daur of the above died Nov 14 1789, aged 5 years
Burials, Ballast HillsJane d/o William & Alice Halbert bap 7 Oct 1787
Baptisms, Castle Garth Chapel - Family Search