I don’t think it is so simple to understand what Justin Martyr believed about the Eucharist.
In Chapter 65 he wrote:
“…There is then brought to the president of the brethren bread and a cup of wine mixed with water; and he taking them, gives praise and glory to the Father of the universe, through the name of the Son and of the Holy Ghost, and offers thanks at considerable length for our being counted worthy to receive these things at His hands. And when he has concluded the prayers and thanksgivings, all the people present express their assent by saying Amen. This word Amen answers in the Hebrew language to γένοιτο [so be it]. And when the president has given thanks, and all the people have expressed their assent, those who are called by us deacons give to each of those present to
partake of the bread and wine mixed with water over which the thanksgiving was pronounced, and to those who are absent they carry away a portion.”
He goes on in Chapter 66:
“…For not as common bread and common drink do we receive these; but in like manner as Jesus Christ our Saviour, having been made flesh by the Word of God, had both flesh and blood for our salvation, so likewise have we been taught that the food which is blessed by the prayer of His word, and from which our blood and flesh by transmutation are nourished, is the flesh and blood of that Jesus who was made flesh. For the apostles, in the memoirs composed by them, which are called Gospels, have thus delivered unto us what was enjoined upon them; that Jesus took bread, and when He had given thanks, said, This do in remembrance of Me, Luke*22:19 this is My body; and that, after the same manner, having taken the cup and given thanks, He said, This is My blood; and gave it to them alone…”
newadvent.org/fathers/0126.htm
The term ‘transmutation’ is referring to the change occurring with digestion after the elements are consumed and not referring to a change before they are eaten. He does call it flesh and blood, but he doesn’t specify a literal conversion or a symbolic representation. He could mean by this that it literally converts or that it represents and carries the meaning of flesh and blood. For example pointing to a piece of paper with an image and stating “this is a flower” when in fact you are merely pointing to a piece of paper which did not undergo a conversion. I wonder if sometime in the distant future a later civilization will uncover the 1980’s “This is your brain; this is your brain on drugs” commercials and believe that we thought eggs were actually brains.
To find out more about whether Justin Martyr thought bread and wine were not common food and common drink because they represented something much more meaningful or whether he thought an actual conversion took place, we need to look to what else he wrote about the topic. In Chapter 26 of this same writing he sharply criticized the mystery cults for possibly eating human flesh: “And whether they perpetrate those fabulous and shameful deeds — the upsetting of the lamp, and promiscuous intercourse, and eating human flesh— we know not;…”
In his Dialogue with Trypho he wrote about the Eucharist numerous times. He wrote:
Chapter 70: “…Now it is evident, that in this prophecy [allusion is made] to the bread which our Christ gave us to eat, in remembrance of His being made flesh for the sake of His believers, for whom also He suffered; and to the cup which He gave us to drink, in remembrance of His own blood, with giving of thanks…”
newadvent.org/fathers/01286.htm
Chapter 117: “…Now, that prayers and giving of thanks, when offered by worthy men, are the only perfect and well-pleasing sacrifices to God, I also admit. For such alone Christians have undertaken to offer, and in the remembrance effected by their solid and liquid food, whereby the suffering of the Son of God which He endured is brought to mind,…”
newadvent.org/fathers/01288.htm
These quotations seem more likely show a symbolic understanding than showing an indication of a concept that would later be called transubstantiation. The concept of transubstantiation had not yet developed by his time and is not clearly evident in his writing.