Larry
Checking the Ib you state with Jim Halley's lists in Aeromilitaria, he states the following Rxxxx were Mk Ib
R6713, R6761, R6762, R6770, R6776,
R6809, R6833, R6882, R6889, R6890,
R6897, R6904, R6908, R6911, R6912,
R6917, R6918?, R6919, R6923, R6924,
R6957, R6958, R6960 (total: 22 or 23)
In the Xxxxx range I have only found the following
X4231, X4257, X4272, X4280, X4342,
X4561 (total 6)
Not verified in these lists are "your"
R6778 and R6888
The funny thing is that if we include R6778 and R6888 we do get the total of either 30 or 31, but I rely on Halley, what do you rely on?
Cheers
Stig