pos is an index to a 10-element array; so it only needs to be a 'char'; not an 'int'
Most (all?) 8051 compilers will use 16 bits for an 'int' - which is a significant unnecessary overhead when you don't need it.

Similarly, you are only using positive values; so it can be 'unsigned'
The 8051 has no native instructions for signed arithmetic so, again, it's an unnecessary overhead when you don't need it:

unsigned char pos;          //position within array
unsigned char Length = 0xA; //Length of frame, excluding SOF and Checksum

Note that I expressed some doubt above about whether your compiler might use 16 bits for an 'int'. This is because the 'C' language does not define specific sizes for the different types.
Therefore it's best to avoid the standard language types, and define your own of known size & signed-ness; eg,

typedef unsigned char U8 ; // Unsigned,  8 bits
typedef signed   char S8 ; // Signed,    8 bits

typedef unsigned int  U16; // Unsigned, 16 bits
typedef signed   int  S16; // Signed,   16 bits

When you start using a new compiler, you have to look-up in the Manual to find the specific type to use in each definition.
You would put these in a header file, and just #include it everywhere...