If the serial numbers are sequential, I'd guess around 50 blue face VH4S amps, based on my serial number and its date of production. I believe I've seen at least serial number 50 (0970050, I think) for blueface VH4 mono heads, too. If the numbers are sequential, then I'm pretty sure the VH4S and the VH4 heads were numbered separately, because I've seen blueface VH4 heads with higher serial numbers than mine VH4S head, but the VH4 head was made in the prior year.