$(2^{k}-1)*10^{d}+2^{k-1}-1=ec(k)$, where d is the number of decimal digits of $2^{k-1}-1$, are numbers formed by the concatenation base 10 of two consecutive Mersenne numbers. With Pfgw I found that $ec(43s)$ is probable prime for the following values:
$(2^{215}-1)*10^{65}+2^{214}-1$
$(2^{69660}-1)*10^{20970}+2^{69659}-1$
$(2^{92020}-1)*10^{27701}+2^{92019}-1$
$(2^{541456}-1)*10^{162995}+2^{541455}-1$
My guess is that everytime $ec(43s)$ is probable prime, then $43s$ is of the form $41*m+r$, with r in the set $(1,10,16,18,37)$. Can somebody find another $ec(43*s)$ probable prime?