"her eyes fell only on the butcher with his tray"

The white space between the lines of this paragraph seem to be crammed with hidden comments, crossed and double-crossed with meaning. You could almost do a group-read on this paragraph by itself! But as we are expecting Mr Elton to return any moment, I'll confine my questions to that cryptic friend who helped him on his way.

In the first place, Emma sees his tray. Now, am I right in thinking this is an enameled iron tray, largish, rectangular, between two and six inches deep, possibly with a grooved bottom (for drainage), to display cuts of meat in?
And if so, is he holding it in his hands? Is it on his cart?
That Emma's eye can fall upon it, suggests that it is not in his shop, and anyway, wouldn't the butchers shop be a little out
of the main way, near the cattle yards, with his block, hooks and shambles by a stream, out the back? There was a law against butchers setting up shop on the highway in medieval times - was it still in force? Was the main street of Highbury a highway, anyway?
I am wondering about his customers, too. When the delicate porker was killed at Hartfield, would the butcher have been called in to do it, or would the servants at Hartfield have managed it themselves? Emma sent the Bates a whole forequarter, and her father fretted about which cuts would be butchered from it - Would it have been Patty or the butcher that took care of that? We know that Patty salted the ham herself, but would the butcher have provided that service? Or was the butcher more like our modern butchers, who bone and slice the product they sell you, rather than offer you their services when you have obtained the meat for yourself?