I have 3 Compostelas tucked away somewhere in the attic. None of them implied 2 stamps onwards from Sarria. But then again, my last Compostela was obtained in 2015. Seems that the PO is a bit more hardcore these days. And as said earlier, just get a piece of paper for your last stamps; They will accept it.
But by all means, a Credential is a very nice remembrance of your last days of walking, so just get another one to finish it off. Reminds me of a story from the VdlP in 2013:
An old Austrian farmer, in his 70-thies, strongly religious, was walking with terrible blisters the whole way. It was blood and other fluids all over the place. I had to take care of his feet on several occasions, but to no avail. His only purpose of walking was to obtain a Compostela.
However, (The VdlP is 1005 kms...), 60 kms before Santiago, he had to go to the hospital, and the doctor said strickly that if he walked one step further, his feet would be in jeopardy. So he took a taxi into Santiago.
At the PO, he explained his purpose and condition, and told them he had only lived through this ordeal in order to get his all-important Compostela, and he told them: "I will take off my shoes to show you the thruth, because I cannot do this again.".
The volunteer said: "Don't bother: I believe you".
He got his Compostela.
Edit: Good old Austrian farmer Guntram: He really made his wish come true. So happy for him, having watched and helped with his sufferings for a month. The incredible was that he kept our speed of 20-25 kms/day. How it must have hurt...