I can also find no marriage for George Lobb to anyone named Mary that would account for Rebecca's baptism so there are at least a couple of possibilities.
1. the marriage took place outside of Cornwall. I can find nothing in FamilySearch but there are many Devon Parishes that are not represented on Familysearch. There are those who would dispute my including this as a possibility but then I have a record of Francis Trewheela from Ludgvan being married (wnd wife) to Elizabeth Crymes at Buckland Monachorum in 1756. He returned to Cornwall to add three more children to his family before being 'invited' to attend the Assizes where he was sentenced to transportation to America in the 1760s.
2. the record of the marriage is lost through either damaged registers or by not having been entered in the register in the first place.
3. the name of the mother has been entered incorrectly with the baptism of daughter Rebecca.
4. the father's name is the one entered incorrectly.
Options 3 and 4 may be the more likely but then there can be no guarantee with any option.
If the mother's name is incorrect at the baptism then perhaps the 1749 St Keverne marriage of George Lobb and Elizabeth Koom might be worth looking at. That would off potential siblings for Rebecca of Elziabeth (1750 St Keverne), George (1752 St Keverne) and Mary (1762 St Keverne).
On the other hand, if we concede the possibility that the father's name is incorrect at Rebecca's baptism then it becomes more interesting. In that case we are looking at your John Lobb and Mary Trevarthen marriage. And in that family there is a fairly significant gap between the baptism of daughter Mary in 1753 and son John in 1758.
I would be taking a much closer look at the family of John and Mary I think.
Having said that ................. I have just downloaded a very interesting Will!
But before I expand on that I will layout the family of John and Mary Lobb:-
John Lobb and Mary Trevarton were married 6th January 1748 at Crowan
1. Jenifred daughter of John Lobb and Mary his wife baptised 15th May 1749 Crowan
2. Henry son of John Lob and Mary his wife baptised 10th March 1750 Crowan
3. Mary daughter of John Lobb and Mary his wife baptised 25th February 1753 Crowan
4.
5. John son of John Lobb and Mary his wife baptised 31st December 1758 Crowan
I have deliberately left a space between Mary and John for the time being. But before I move on I should note that there are just FIVE Lobb children baptised at Crowan between 1740 and 1770!
Now to the Will - John Lobb of Crowan 1785
Will Written 4th March 1772
Will Proved 12th May 1785
First 'to my dearly beloved wife
Jane ......'
Secondly '.... unto my daughter Jenifer now the wife of Richard Pearce ....'
Thirdly '.... unto my daughter Mary ....'
Fourthly ' .... unto my daughter
Rebecka .....'
'I give unto my son John Lobb .....'
'All the Rest Residue and Remainder ......... unto my son Henry Lobb whom I do make ......... Executor'
Most interesting don't you think?
So, the first thing to note is that there were just the five Lobb baptisms at Crowan over 30 years - that is quite an important point.
The next point of note is that John named his wife as JANE ..... I will return to that point in a moment ....
Now, my list above shows 4 of the 5 Lobb baptisms and as you can see each of those children are mentioned in the Will. The one child for whom I have not yet provided a baptism is daughter REBECKA:-
Rebecca daughter of George Lobb and Mary his wife baptised 22nd February 1756 CrowanThere can be but one conclusion and that is that this is the family you are looking for. The fact John names his wife as Jane is that he was widowed and then remarried although that would not be apparent simply by reading the marriage records. There are just a couple of records needed to fill in the gaps but I have to caution that there are two possible burials for the first wife.
1. Mary Lobb was buried 20th February 1764 Crowan
2. Mary Lobb was buried 14th June 1767 Crowan
And the marriage (at Crowan):-
John Lobb of Crowan labourer and Jane Heys (Keys?) of Crowan spinster were married by Banns 8th September 1770 - Witnesses:- John Bellhouse, Robt. AbbittI think that should help you along a little bit.
CT