Vertical centering of display number in @CentredNumberedDisplay when using a @Tbl

Consider the example below.  I'm trying to figure out how to vertically
align automated display numbers, without much success.  Any help
appreciated.

@SysInclude { doc }
@SysInclude{ tbl }
@SysInclude{ math }
@Doc @Text @Begin
@Heading { Vertical align display numbers with a table }
@LP
@CentredNumberedDisplay
@Tag { eqnMyFirstEqn }
@Math { a = 1 + 2 }
No problem, the number on the right is vertically centered.  But below, the
number on the right seems to be vertically aligned to the top of the
display.  How can I centre its vertical alignment?
@CentredNumberedDisplay
@Tag { eqnCombinatorics }
@Tbl
rule { no }
aformat { @Cell A | @Cell B }

{

@Rowa

A { @Math { b = } }

B { @Math { 3 + 4 } }

@Rowa

A { @Math { c = } }

B { @Math { 5 + 6 } }

@Rowa

A { @Math { d = } }

B { @Math { 7 + 8 } }

}

@End @Text