I had thought this was a first generation Annihilator bolt but looking at it again it isn't.
I think this must have been the direct feed location used for a brief period after Mac-1 stopped having South Bay Arms manufacture their 1st generation Annihilators. This would have been right before the Annihilators MK1 (common Annihilator style) came out and was manufactured by A++ (Tim's next machinists). This would have been in (I think) 1987. Likely right before the PMI1 came out and became the standard p series length centerfire bolt.
I think it's for the same length feed as the second bolt down in this picture:
Sheridan pump bolt comparison picture. From top to bottom:
*Cooper -T standard pseries length
*Bizarre length Mac-1 bolt?
*1st gen annihilator bolt (will not fit kp or p series)
*cooper t kp2 bolt
*kp3 bolt (bbs in different spot than kp2)
*Masse Venturi kp3 bolt
Hope that makes sense and helps.