And with our improving product performance, we expect further growth in the installed participation base and revenue per day in June quarter and beyond. Next our new unit sales continue to increase on a quarterly sequential basis with 600 more units sold for replacements on casino floors in US and Canada compared with the December 2011 quarter. Contributing to this improvement was the successful launch of the Bluebird2e cabinet which adds emotive lighting to the Bluebird2 cabinet. We shipped more than 1500 of these units in the March quarter.
And that's particularly noteworthy, we improved product sales gross margin by 320 basis points over the prior year to 52% which is up 170 basis points sequentially from the December quarter. Ongoing benefits from our cost containment and continuous improvement initiatives more than offset the roughly $1100 decrease in average selling prices that primarily reflected a greater mix of lower price VLT sales, the impact from higher discounts given our larger volume orders, fewer premium for sale of units and the continued impact of a competitive market place.
Next our ongoing cost containment efforts coupled with the savings program instituted earlier this fiscal year resulted in a R&D and SG&A expenses in aggregate being $8 million lower than the prior year period. Next the operating leverage inherent in our balance model was once again validated, as we increased unit sales volumes roughly 480 basis points increase in operating margin on a quarterly sequential basis.
The $11 million quarterly increase sequential and gross profit contribution when tax affected, largely dropped straight through the bottom line. I think it is plainly evident that since we implemented the organizational realignment and revised product commercialization plan early in fiscal 12, WMS achieved measurable progress in our core business, but we still have much to achieve and our opportunities to assist customers and grow remains plentiful. Most importantly the flow of new game approvals is back to a normalized rate. This is a significant milestone as it begins to revalidate our historical consistency for bringing innovative new products to market and these new games are expanding our portfolio of the unique innovative gaming features in a more diverse range of math models.Read the rest of this transcript for free on seekingalpha.com