Bless, Father Silouan!
Yes, the Optina Cross CAN be an Old Believer-style baptismal Cross owing to its style, the writing on the back etc.
The Old Believers represented the ancient Russian usages which persisted even in the Nikonian era unabated.
For example, everything about St Seraphim of Sarov could be said to have been "Old Believer" except for his three-finger Sign of the Cross AND his insistence on kneeling for long hours (Old Believers prefer not to kneel and do not encourage it).
St Seraphim had a large bronze "Old Believer style" Cross on his neck, a Lestovka and recited daily the 150 Hail-Mary Rule of the Theotokos as Old Believers always do!
Can you imagine? Some ill-advised people call that rule of 150 Hail Mary's a form of the "Rosary!"
How misguided can some people get?

Alex