cwhitmore,

Welcome!

C2C is on our development list, but unfortunately I don't have a date to give you for when it will be available.

I know it's tempting to use the "standard" single crochet stitch (double crochet if you're in the UK) for C2C, but unfortunately standard stitches are rectangular and not square. Depending on how large your project is, the results might not work well, so it's not recommended to use this for C2C patterns.

Grace,

Welcome! 

Do you have a picture of the bed spread that you can post? Have you tried searching by image?

I can refer you to two sites I know offhand that might have what you're looking for, and both are free. There's the Antique Pattern Library (this link will bring you directly to the crochet category, but you can look at other categories, of course):
https://antiquepatternlibrary.org/html/warm/crochet.htm

And then there's this one:
https://freevintagecrochet.com/

(Categories are at the bottom of the page, or you can go directly to the only match I could find that seemed relevant, afghans and blankets, linked below.)
https://freevintagecrochet.com/index.php/afghans.html
I did a pretty intensive search for "cross," and found only two matches from the Free Vintage Crochet site, which weren't close to celtic crosses. I did find one sort-of match on the Antique Pattern Library site, which is from The Workbasket, 1947 for an altar cloth with crosses, so perhaps this is adjacent to what you're looking for, or could be reworked into a charted filet replica?
https://antiquepatternlibrary.org/html/warm/I-WB127.htm
(The free downloadable PDF for this is in the upper left-hand corner, where it says PDF. You can also look in the book by clicking or tapping on the thumbnail images of any of the pages below the title and description, which will enlarge them, and you can see if this is what you want.)
While there were over 160 matches to "cross," most of them were cross stitch related.

If the above pattern isn't close (and it probably isn't, because what are the chances?), please do avail yourself of these resources; just because I didn't find anything close doesn't mean it's not actually there. It could be that there's nothing in the text or pattern name that mentions crosses, but is still in the correct book. And it could also be that it's not there, unfortunately.

Having personally done a lot of searching for older patterns, I know it's really hard to find anything specific that's vintage, as so much has been lost over the years, and especially from the 1920s and earlier, when apparently no one thought to archive anything.  And another thought that occurs is your Grandmother freehanded it, as a lot of women back then probably did.

I wish you the best of luck with your search and hope you can find what you're looking for!